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Why is my car making a knocking noise?

Ellie Dyer-Brown, 11 months ago

5 min read

  • Advice
  • Maintenance
diagnose-IT-s2-knocking

This guide will explore the various causes of the knocking noise, dependent on where the noise is coming from and when it is occurring

Any unusual noise coming from your vehicle will ring alarm bells in your head. Cars often seem to have their own language, and the noises they produce are one way they communicate their problems to us. 

A knocking noise is no different and is certainly not one to be ignored.

Guide Contents:

Why is my engine knocking?

Car makes knocking noise when accelerating

Knocking noise under the bonnet


Why is my engine making a knocking noise?

A knocking sound from an engine can be the result of a number of issues, it could even be something as small as poor-quality fuel.

No matter how small the issue, a knocking sound is not something you should dismiss.

Engine knock is generally less of an issue with newer cars, as their injectors, air/fuel mixture ratio and timing is all controlled by computers. Most cars also have a knock sensor. The sensor is engineered to pick up on any engine knock, which then alerts the ECU to correct the issue. Thus, on newer cars, a knocking noise may literally be the result of a faulty knock sensor.

Engine knocking may also be the result of:

An engine noise that appears to be coming from the bottom of the engine is more likely to be because of damaged rod bearings than anything else.

Your cambelt should be replaced every 40,000 to 100,000 miles - if your car is approaching this distance and you haven't recently changed your cambelt, this could explain the knocking sound.

Wheel bearings, on the other hand, are a very common cause of knocking - particularly when the vehicle turns left or right. If the noise sounds like it might be coming from one of your wheels, you should get it checked as soon as you can, because if your wheel bearing breaks completely while you're driving it could have disastrous consequences.


Knocking noise when accelerating

A knocking noise when you accelerate may well be an engine worn out with low oil pressure or a piston slap. Regardless of which one is the issue, you must have the vehicle looked at by a professional.

The noise may also be occurring because the fuel grade you are using is lower than manufacturer specifications, in this case the knock is more rapid.

If this knocking develops into more of a humming noise when accelerating, you may have a failing wheel hub bearing on your hands. Engine issues can quickly become serious so deal with them promptly to avoid any excessively expensive repairs.


Knocking noise under the bonnet

If your car is making a knocking noise under the bonnet it is likely a sign of transmission problems or a worn crankshaft.

The noise can also occur if your air-fuel mixture is incorrect or if there is a lack of lubrication in the upper cylinder head. Your best bet under these conditions is to visit a garage for a full diagnosis and subsequent repairs.


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