We’ve already addressed reasons why your car engine can be noisy or obnoxiously loud while accelerating. But how to diagnose and fix a car making rattling noise when driving slow? Take it as a cry for help – your car is asking you to take quick corrective measures to keep the problem from compounding.
In this article you’ll learn how to diagnose the problem, top ways to fix it and how much it’ll cost ya.
1. Loose Exhaust Mounts
If you hear a rattling under car, chances are the exhaust system is to blame. The loud noises could mean the exhaust pipe comes into contact with other metallic parts as the car moves or vibrates.
Worn exhaust mounts, including brackets and connectors, allow the exhaust pipe to bang against other car parts. The placement of exhaust systems exposes them to debris, heat, road salt, moisture, and impact damage.
As a result, exhaust pipes have a higher deterioration rate than other car parts. Exhaust mounts are especially vulnerable. Bolts and nuts become loose, corroded, or knocked off, causing the exhaust to hang loose and move out of alignment.
If you have loose parts in the exhaust pipe, you can count on them to rattle whenever the engine runs. The exhaust pipe will likely fall apart when the mounts fail due to physical damage or corrosion.
Generally, you’ll notice mild rattling noises when you start the car and leave the engine idle for a while. The rattling will intensify as you accelerate or drive over bumpy surfaces.
While a rumbling or loud rattling noise is the most common sign of a faulty exhaust system, a quick look can help confirm it. Check your exhaust system for signs of rust and damage. Pay attention to your engine performance if the exhaust pipe is damaged. You may notice a marked decline in engine power and car performance.
Replacing the damaged exhaust mounts is the most effective way to fix the rattling noises. If you take quick action, you only need to replace the exhaust hangers, connectors, and brackets. Otherwise, you may have to replace the entire exhaust system. Replacing an exhaust system costs $300 to $1,200, depending on the car’s make and model.
2. A Failing Catalytic Converter
A catalytic converter is the globular part of your car’s exhaust system. It converts harmful gases such as carbon monoxide to carbon dioxide and water to lower the impact your car’s exhaust emissions on the environment.
The catalytic converter comprises a steel casing with a ceramic honeycomb structure lined with platinum, palladium, and rhodium catalysts.
Like other car parts, the catalytic converter is susceptible to damage and wear and tear. Residue buildup in the catalyst chambers causes the honeycomb comb structure to crack and disintegrate the pieces. The gases escaping through the crack and the small pieces moving around cause rattling noises.
You’re more likely to hear the rattling from the catalytic converter while driving at low speeds. Other signs of a compromised catalytic converter include an illuminated check engine warning light and noxious, dark-colored smoke.
A rattling catalytic converter is a serious problem that requires immediate attention. When your converter is compromised, it sets the stage for additional engine trouble. You will notice the car sputtering during startup, losing power when going up or down a hill, and impaired peaking ability.
There are several ways to fix a rattling converter, including:
- Filling up with high-octane gas every third or fourth trip to the gas station.
- Cleaning the converter with Cataclean, sodium hydroxide, or lacquer thinner.
- Replacing the faulty catalytic converter with an OEM or a universal fit. (video tutorial)
3. A Faulty Heat Shield
The heat shield is an aluminum or copper sheet mounted over your car’s catalytic converter using nuts and bolts. These shields protect the cab floor from heat damage.
Catalytic converters generate a lot of heat while the car is in motion. The typical operating range is 500 to 1200°F, which is hot enough to extensively damage your car.
If the heat shield comes loose, you may notice a rattling noise coming from underneath the floor cab. The constant engine vibration, rust, corrosion, or physical damage may loosen or damage the heat shield’s fastening mechanism.
Corroded bolts and nuts may fall off, or an impact on the undercarriage may mangle the metal flanges. Loose or missing bolts means the heat shield is no longer firmly secured to its mounting. The engine vibrations cause it to bang against the car’s undercarriage, creating a rattling noise.
The rattling noise from a faulty heat shield is more noticeable when driving at low speeds. The intensity and tone of the rattling changes as your increase speed. You’ll notice the engine bay getting noticeably hotter after the rattling begins.
It’s advisable not to ignore a loose or faulty heat shield since it may expose your car to extensive heat damage. The car’s vibrations may shake all the bolts loose and cause the heat shield to fall off.
The plastic components of your car may start disintegrating as there’s nothing to protect them from temperatures above 1,000°F. The first indicator of a missing heat shield is a strong burning smell in the engine bay. By then, it’s already too late, and the damage is done.
A faulty heat shield is easy to fix. You only need to replace the missing or damaged nuts and bolts and secure the shield to its mounting. You may need to replace the heat shield if it’s badly damaged or corroded. Heat shields are relatively cheap and affordable. Replacing a worn heat shield will cost you $50 to $150, depending on your dealership and car’s make and model.
4. Worn Engine Mounts
Combustion engines produce a lot of energy and vibrate a lot. The vibrations and the impact of traveling at high speeds can loosen the engine mounts and bolts.
Car manufacturers use engine mounts to secure the car to the chassis and absorb road shock and vibrations when the car is running. Engine mounts also enhance driving comfort and protect the engine from damage. Most cars have two or three engine mounts and one to hold the transmission.
When your car’s engine mounts start to wear out, you’ll notice loud rattling noises at ignition and during acceleration. Since broken engine mounts can’t securely hold the engine in its compartment, the car will vibrate and rattle while idling. The rattling noise and vibrations caused by worn engine mounts disappear when you shift to neutral.
It’s best to seek professional help if you suspect the engine mounts in your car are worn, weak, or damaged. A professional mechanic is better placed to diagnose and fix the problem.
While worn engine mounts aren’t a critical problem, they leave your engine vulnerable to vibrations and road shock. If you don’t fix the loose or broken engine mount problem, your engine could sustain extensive damage and start malfunctioning.
It’ll cost you $200 to $600 to replace the engine mounts on your car. The final cost comes down to engine accessibility. Expect to pay more if your engine is hard to access. Overall, much cheaper than a complete engine rebuild.
5. Loose Brake Components
Stopping a speeding car takes a toll on your braking system. Each time you apply the brakes, the brake pads apply immense pressure on the brake motor. The resultant friction brings the car to a stop. If you notice a rattling noise after taking your foot off the brake pedal, your braking system could be faulty.
Among other problems, a rattling noise indicates the presence of loose components within the brake system. The rattling may result from a warped motor making uneven contact with the brake pads. Missing brake hardware or incorrect adjustment may also cause a rattling noise at low speeds.
Your system may be missing caliper brackets, anti-rattle clip, or wheel bearing, or they might have sustained some damage. Alternatively, you may have dirty brake caliper slides. Dirty caliper slides keep the brake pads from functioning properly, causing the brake caliper to stick. This may also cause a clattering noise.
Given the critical role the braking system plays in your driving experience, it’s best to have a professional fix your noisy brakes. You will need to fix the loose component and replace the warped drum or rotor to fix the rattling noise.
You’ll need to clean the brake calipers and replace missing brake system components such as anti-rattle shims, anti-rattle clips, and brake lining, or adjust them correctly.
A complete brake system repair, including pad replacement and installing new calipers and rotors, will likely cost you $300 to $800 per wheel. The overall cost depends on the car’s make and model and the parts to be replaced.
6. Low Oil Pressure
Oil is the lifeblood of your engine. It keeps all the moving parts well-lubricated and lowers the temperature to keep the engine humming nicely. However, you must keep the oil well-topped up and ensure it doesn’t fall below a certain minimum threshold.
You may hear a rattling noise from your engine when the oil levels dip too low. The ticking or rattling sound results when the engine does not have enough oil to work with. Low oil levels lower the oil pressure and force the oil pump to pump a mixture of air and oil into the engine, causing extensive damage.
Dwindling oil levels lower oil pressure and starve the car engine, leading it to seize up and die. Driving without engine oil is the fastest way to ruin a car engine.
Topping your engine with the right oil will quickly fix the low-pressure problem. However, adding engine oil is a temporary fix; you’ll need to address the root cause of dropping oil pressure, such as oil burning or leaks. The solution may range from drain plug gasket replacement to extensive engine repairs.
The cost of repairing an oil leak ranges from $100 to $2,000. The overall price depends on the source and location of the leak, and the type of car your drive. For instance, it may cost you $100 to $500 to repair an oil pan, but you can replace an oil filter for $30 to $70.
7. Leaking Transmission Fluids
Low transmission fluids may be responsible for any rattling noises coming from the gearbox area of your car. A well-lubricated transmission operates noiselessly. But a poorly lubricated transmission will produce rattling noises at low speeds.
Besides providing lubrication, the transmission fluid also helps to cool your car’s transmission. Low fluid levels may cause the transmission to overheat and produce a rattling sound.
If you notice rattling sounds coming from the gearbox area, take immediate action to avoid ruining your transmission:
- Stop the car and switch off the engine
- Leave the car to cool for a while
- Check your transmission fluid levels
- Top up the transmission fluid to the required level
- Schedule an appointment with your mechanic to repair the leak
Cracked fluid lines, damaged pans, degraded pan baskets, broken seals, and worn torque converters are the main culprit behind leaking transmission fluid.
The average cost of fixing a simple transmission fluid leak is $150 to $200. You can replace the drain plugs, pan bolts, fluid lines, gaskets, and seal at this price. It may cost north of $1,000 to replace a torque converter.
8. Bad sway bar bushings
Sway bars prevent rollover when you turn. The sway bar BUSHING separates the sway bar from the car’s body to reduce noise and friction.
When they break or wear out you will experience poor hadling and a variety of noises: knocking, rattling and squeaking coming from under the car. Especially when going over bumps and taking turns.
There is no fix for bad sway bar bushings other than replacing them. This video demonstrates the noise caused by bad sway bar bushings and how to replace them.
9. Bad control arm bushing
The sway bar bushing connects to the control arm bushing. So the same problem of the car making a rattling noise when driving slow or turning can happen.
A control arm can break or wear out and it will create rattling, clunking and other sounds. These are caused by the bushing getting knocked between the frame and the control arm. Vibrating steering wheel is a tell tale sign, as well as a steering wheel that wanders from left to right.
This video demonstrates how to check for bad control arm bushings on the front of the car.
10. Piston Slap
A piston slap is the most disturbing cause of a rattling noise while driving slowly. It’s the equivalent of a grim reaper for car engines – it indicates that one or more pistons in your engine are failing. Since a piston slap is a direct result of a worn piston and cylinder, it only happens in old cars.
But what is a piston slap? A piston slap is an unwelcome movement whereby the piston rocks side-to-side within its cylinder. Typically, there’s a snug fit between a piston and the cylinder. There’s almost no clearance between the cylinder walls and the pistons. Since the piston only needs to move up and down, there’s no need for a gap.
With time, heat and friction wear down the piston and cylinder walls, creating more room between the two. Due to the additional space, the piston begins to rock back and forth within the cylinder. The skirt (the piston’s cylinder wall) also slaps the cylinder wall.
The rattling occurs when the piston’s skirt clangs against the cylinder walls. This disturbing rattling sound is louder during idling and the overrun (when you ease off the gas pedal and the RPM goes down).
Piston slaps are more common in some performance cars. Such cars use engines with aluminum pistons or cylinder blocks because they’re lighter and easier to cool. Unfortunately, they’re not as strong or durable as pistons or cylinders made of cast iron.
Should You Worry About a Piston Slap?
Save for the uncomfortably loud rattling; you can still drive your car. Unfortunately, you don’t need to worry about piston slap ruining your engine because it’s a form of engine damage.
I’d recommend keeping an eye on your car’s exhaust gases. A piston slap may cause your piston rings to wear out faster, leading to an oil leak in the engine.
Any oil leaking into your car’s engine cylinder is burned together with the fuel and air mixture. An engine that’s burning oil emits a characteristic blue smoke.
You should replace the pistons and the rings immediately if you notice your car emitting blue smoke. Failure to do so only invites additional problems, such as a damaged catalytic converter.
Can You Fix a Piston Slap?
Save for dismantling your engine to replace the affected piston; there’s no easy way to fix a piston slap. Piston slap may affect just one piston, but it takes an entire engine rebuild to fix the problem. You’ll need to replace all the pistons and piston rings in the engine block.
First, your mechanic will need to do a compression test, which will carry an extra charge, to assess the extent of the damage. The engine block is virtually unusable if the cylinder walls are extensively damaged.
You’re better off scrapping the engine and getting a new one. Or consider buying a new car altogether. An engine rebuild is likely to set you back $2,500 to $4,000 but can go as high as $8,000.
Final Thoughts: Car Making a Rattling Noise When Driving Slow
Rattling noises spell nothing but trouble for your car. You should take immediate action if your vehicle makes rattling noises while driving at low speeds. Timely intervention lets you fix the issue before it snowballs into extensive repair needs that’ll cost you a fortune.