8 Places Your Car Rattles While Accelerating (And Why It Happens)

(Updated on December 27, 2021)

Road trips are supposed to be fun, but a loud ride hinders the experience. The only thing more annoying than your kids chanting “are we there yet” is a constant rattle that won’t seem to go away.

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Not only are rattles annoying, they can be a sign of a much more serious issue. It’s important to locate the source of the rattle. Once you find the rattling sound, you can make an informed decision on whether it’s safe to deal with the noise for now, or an urgent repair is warranted.

Why Do Old Cars Rattle?

Cars tend to rattle more as they age due to previous repairs, simpler manufacturing practices, vibration, and heat cycles.

Loose Fasteners

Subaru timing

Vehicles are bolted together using thousands of parts made out of different materials. The vast majority of the vehicle is put together using fasteners such as bolts, nuts, studs, and screws.

As you drive, your vehicle is subjected to intense and continuous vibration from the surface of the road, the engine, and even from the audio system. Most fasteners are designed with a locking mechanism to keep their components tightly coupled. Under normal circumstances, these fasteners are designed to never loosen over time.

However, there are many reasons a bolt or nut could back out and eventually cause a rattle. Some possible causes are age, excessive vibration, excessive heat, and improper torque specs after repairs are performed.

Vibration

Vibration is the enemy of fasteners. Given sufficient vibration, nearly all components will come apart without using some type of thread adhesive.

Engineers keep these vibrations in mind when manufacturing a vehicle, but sometimes unforeseen circumstances arise. If you have a worn engine mount or bad harmonic balancer, the vibration could be enough to loosen some bolts throughout the vehicle.

Heat

engine overheating symptoms

As components heat and cool, they expand and contract at different rates depending on the material. If a component heats up too much, it may warp the component, or make it easier for a fastener to back out. For instance, some types of thread locker can only be loosened when high heat is applied.

Improper Torque Specs

A torque specification or “torque spec” is the factory specified twisting force used to tighten a bolt upon component installation. This force is usually measured in Newton-meters (Nm) or foot pounds (ft-lbs).

When a component is installed too loosely, the locking mechanism on the fastener may not have enough bite into the surface of the component. They may stay together in your garage, but as soon as you start driving down a bumpy road, the two components may begin to separate over tens of thousands of miles.

When a component is installed too tightly, you may pass the fastener’s yield strength. Yield strength is the amount of force a bolt can take before it permanently deforms. Permanent deformation may weaken the fastener and cause premature failure. Over tightening may also strip out the threads in the bolt hole.

Interior Trim Panel Fitment

door card

Interior panels may deform or become more brittle as they age. They also tend to develop a looser fit when they have been removed and reinstalled several times.

Some manufacturers put a lot of thought into interior panel fitment and noise. On newer and higher end vehicles, you will find soft materials such as felt behind contact points. Wiring harness may be secured so they don’t move.

Cheaper and older vehicles may have spared this expense, which results in more plastic on plastic or metal on plastic contact. These noises can be heard in the cabin, particularly over rough terrain or while playing loud music with deep bass.

Age

Component wear between the fastener and mating surface after several removal and installation procedures may contribute to loose bolts.

Age-related fastener failures also may happen when one or more of the above circumstances occur at the same time over many thousands of miles.

In other words, some combination of heat and vibration over a long period of time.

8 Reasons Your Car Makes a Rattling Noise

1) Upon Startup

timing chain noise

A loose timing chain tensioner may cause a rattle right on startup that goes away after a couple seconds. Sometimes this is the sound of normal operation on louder engines, but many times this indicates a problem.

If your starter is on its way out, it may be struggling to fully retract the pinion gear, even after the engine has already started. This gear may be spinning and rubbing against the flywheel.

2) At Low Speeds

bad coil spring symptoms

Sources of low speed rattles are steering and suspension components. Loose wheels are a very serious issue that may also cause a rattle or intense vibration. Always double check your lug nuts after removing the wheels for maintenance.

Low speed rattles may also be present at higher speeds. Sometimes the rattles are only noticeable when you’re going slow because road, wind, and tire noise levels are lower.

3) At High Speeds

steering wheel shakes

High speed rattles could be caused by loose suspension components over bumps or wheel imbalances.

If you notice that the rattle is accompanied by additional vibration, it may be time to pull over and give the wheels, tires, and steering components a thorough inspection.

4) In the Cabin

noise while driving

Interior rattles are often caused by trim pieces, bits of the wiring harness, components within doors, or parts of the dashboard.

You might also want to check your glovebox. It’s pretty easy for loose items in storage compartments to cause a lot of noise simply from the normal vibration of the road.

5) Under the Car

symptoms of a bad catalytic converter

Exhaust systems are a common source of rattles. Exhaust hangers may become loose or damaged over time, allowing the exhaust system to move more than it should.

You may also notice rattling from loose heat shields. Heat shields are thin metal covers that wrap around part of the exhaust to keep heat away from sensitive components.

These components may be the wiring harness, an air intake, brake lines, a fuel tank, or anything else that can’t stand up to the unrelenting heat of a hot exhaust. Most exhaust systems have one or more heat shields.

As catalytic converters age, fuel eats the catalyst and may cause the unit to rattle. If the honeycomb structure comes loose from the walls of the catalytic converter, it may become lodged sideways. This restricts exhaust flow and could even cause a fire.

Bad transmission mounts or other transmission problems may also cause a rattle that comes from underneath the vehicle.

6) In the Engine Bay

Subaru EJ Engine

It can be tricky to find a rattle coming from the engine bay. Engine bays contain a large number of components that are crammed into a tiny space. The part count is even more higher on newer and more complex vehicles.

If you notice a rattle at idle or one that speeds up when you rev up the engine, check the serpentine belt and accessory pulleys first. Make sure nothing is contacting the belts or pulleys.

You can use a mechanic’s stethoscope to help you narrow down the source of the sound. This is particularly useful when it sounds like the rattle is coming from inside the engine.

Make sure nothing is interfering with your radiator fans. Radiator fans and the A/C compressor could be the source of intermittent rattles, since they do not run all the time.

7) At Certain RPMs

high rpm

If your engine seems to rattle at a certain RPM, don’t fret just yet. It could simply be the resonance of the engine that is rattling an old heat shield.

Combustion engines produce sound like the tone of a musical instrument. Since different engine speeds produce different sound frequencies, some of the rev range may produce a tone that causes a rattle due to resonance.

Be sure you don’t confuse rattles with ticking or knocking noises, which can also occur at a specific engine speed. Ticking and knocking are typically associated with more serious engine problems.

8) From the Rear End

bad differential symptoms

If you drive a rear or all wheel drive vehicle, your vehicle has universal joints (or u-joints). These joins allow the driveshaft to reach from the transmission or transfer case to the rear differential.

Rattling from the rear end could be coming from a loose u-joint or a problem with the rear differential. Bad differential mounts may also cause a rattle.

Conclusion

It can be difficult to diagnose the source of a rattle, particularly when the noise seems to vibrate the entire car. Start by narrowing the source down to one area of the vehicle, then see if you can wiggle components around to change the sound.

Tighten any loose bolts in the area according to the manufacturer’s torque specs. If loose bolts back themselves out again, apply some thread adhesive to better secure the fastener.

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