Why Is My Car Making a Knocking Noise?

Ellie Dyer-Brown, 8 months ago

5 min read

  • Diagnostic
Knocking noise blog

WhoCanFixMyCar discusses why your car might be making a knocking noise.

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?

Is it safe to drive with a knocking sound?

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:

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 is £426.76, according to WhoCanFixMyCar’s data.

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.

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.

4. Old spark plugs

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

Spark plugs

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.

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.

Fuel pump replacements typically cost around £343.95.

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.

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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