Why Is My Car Making A Knocking Noise? - WhoCanFixMyCar

Knocking is one of the many noises cars make when something isn’t right. There can be numerous causes, from low oil pressure to transmission problems, some of which are straightforward to fix.

This guide explains the possible causes of your car’s knocking sound and suggests suitable repairs, including average prices where data is available.

Why is my engine making a knocking noise?

A knocking sound from your car's engine isn't necessarily a sign of a serious problem; it could be something as simple as poor-quality fuel. 

Most cars have a knock sensor engineered to pick up on any engine knocks, telling the ECU to correct the issue. Thus, a knocking noise may result from a faulty knock sensor.

Engine knocking can also be caused by:

• Cambelt or chain issues

• Low oil pressure

• Valve lifter failure

• Old spark plugs

• Bad fuel pump

• Worn bearings

1. Worn cambelt

The cambelt is a crucial component responsible for controlling the timing of the internal combustion engine. If it is too loose, damaged or worn, it can cause knocking sounds.

The cambelt should be replaced every 40,000 to 100,000 miles - if your car is approaching this distance and you've yet to change your cambelt, this could explain the noise you’ve been experiencing. Find out what happens when your cambelt is replaced.

The average price of a cambelt replacement on FixMyCar is £706.21.

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2. Low oil pressure

When insufficient oil runs through the engine, components that rely on it for lubrication and protection are left exposed. Connecting rods begin to knock against each other, and if you continue driving, it will eventually cause damage to multiple engine parts.

Finding the right engine oil and topping it up is easy enough, but the low pressure indicates there may be a leak in the system. This guide discusses how to identify and fix various fluid leaks. If, after topping up the oil, you find it running low again quickly, this is a sure sign of a leak.

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3. Valve lifter failure

A valve lifter is a cylinder that sits on the camshaft, transferring motions of the cam lobe through the pushrods and rockers to open and close the valves. Lifters can become clogged when your oil is too dirty or if it is the incorrect grade for your engine; when this happens, you may notice knocking sounds that persist after you have started the engine.

Learn more about how engines work here.

Running a cleaning agent through your engine may solve the problem if a dirt buildup has caused it. If not, contact a mechanic for an accurate price estimate.

Book an engine repair

4. Old spark plugs

Spark plugs are long-lasting, but eventually they wear out. Signs you need to replace them include:

• Check engine light

• Engine stalling

• Struggling to start

• Misfiring

When spark plugs don’t ignite the air-fuel mixture properly, it can cause knocking and misfiring, which sound similar. Your vehicle handbook contains information about how often to change your spark plugs; you usually get at least 30,000 miles out of them.

The average price of a spark plug replacement is £159.17.

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5. Bad fuel pump

Running rich - which means too much air is in the combustion chamber - is a major cause of engine knocking. Several components impact the air-fuel ratio, the fuel pump being one of them. When it malfunctions, not enough fuel reaches the engine, causing it to knock.

A fuel pump replacement costs £339.17 on average.

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6. Worn engine bearings

Engine bearings support the crankshaft and allow rotation when the engine is operating. When worn, they may not offer enough support, creating a knocking sound when pistons rattle against the crankshaft.

Compare engine bearing replacement prices

Knocking when turning

The parts most likely to cause a knocking noise while turning are:

• Track rods

• CV joints

• Anti-roll bar link

Track rods

Track rods connect the steering rack to the steering knuckle, making turning possible. Common symptoms when they become worn or damaged include:

• Erratic steering

• Uneven tyre wear

• Misaligned wheels

• Vibrations and clunking noises from the front of the vehicle

Average track rod end replacement price: £182.93.

CV joints

Constant velocity (CV) joints are flexible joints found at the end of the axle shaft. They allow power from the engine to the wheels to be delivered at a constant speed, even when the suspension is compressed by bumps in the road.

Signs of a damaged CV joint include:

• Clicking or knocking noise when turning

• Grease on tyres

• Car pulls to one side

Average CV joint replacement price: £190.68.

Anti-roll bar link

The anti-roll bar keeps a car stable and prevents the body from rolling when cornering and manoeuvring. It does this by forcing the opposite wheel's suspension to compress during a turn, keeping the body more lateral.

Signs of anti-roll bar link failure include:

• Excessive body roll

• Loose steering

• Poor handling

• Knocking, ratling or clunking noises when turning

Estimated anti-roll bar link replacement price: £80-£200.

Knocking sound when accelerating

The two most likely causes of a knocking sound when you accelerate are low oil pressure and damaged engine mounts.

Low oil pressure

Engine oil lubricates various moving parts, which prevents the build-up of heat and damage from friction. When the oil is low, parts can knock together, and this noise will likely be more pronounced as you accelerate.

Other signs of low oil pressure can include:

• Oil warning light

• Burning smell

• Reduced performance and fuel efficiency

Topping up your engine oil is something you can do yourself (make sure you use the correct oil grade), but if the pressure is low due to a leak, it will require the attention of a trained mechanic.

Average oil change price: £115.02.

Damaged engine mounts

Engine mounts are responsible for securing the engine and absorbing shocks and vibrations.

Signs of worn engine mounts can include:

• Excessive vibrations

• Louder engine noises

• Clunking or banging

• Misaligned engine parts

Estimated engine mount replacement price: £150-£250.

Is it safe to drive with a knocking sound?

Knocking can be a sign of a serious problem with your engine, so you should avoid driving and book a diagnostic test as soon as possible. Using your car when it is making a knocking sound can be dangerous, and you may end up with a more expensive repair bill.

If you have any other issues with your car that you'd like to try to diagnose yourself, check out our guide on self-diagnosing car problems. The more you know about how your car works, the better.

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