Causes of knocking sounds under the car when accelerating

Various extraneous sounds and knocks in the car serve as an alarm signal and are a reason to pay close attention to the car’s technical condition. A knock is a backlash that appears from the natural wear and tear of car parts. This noise is generated from rubbing elements.

The sound of a serviceable car should not cause discomfort. If you hear any unpleasant noises and knocks, you must react to the signs immediately, otherwise, you risk damaging other car parts because of that fault quicker.

How to determine the location of noises and knocks?

To know the right place of a knocking sound, you first need to find out from which side this noise occurs: If it is from behind or in front, to the right, or to the left. 

If you have already identified the approximate location of the noise, it will be much easier to find its right location. Additional info will also help, like: under what conditions the sound occurs and with what it is synchronized.

Let’s find out what parts can cause these sounds when accelerating.

1. A failing water pump

If a thin whistle or a knock small in intensity is heard from the front of the engine compartment, then the water pump is faulty. In this case, you need to check it, and if that is the problem, replace it.

2. Problems with the fuel-air mixture in the engine

A knock that is similar to a loud metallic clatter when the car accelerates indicates detonation (an explosion of the fuel-air mixture in the combustion chamber). Most likely, the ignition is too early.

This often happens if you have filled the tank with low-quality fuel or there is a large amount of soot in the combustion chamber. In addition, the detonation can be caused by engine overheating or bad spark plugs.

The above sound is produced by the engine pistons, which absorb the impact of the combustion mixture explosion. Don’t neglect this symptom otherwise, it will lead to big engine damages that will require expensive repairs or replacing the engine entirely.

3. Failing timing chain

The knocks of the timing, and here we are referring especially to a timing chain can be heard very well. If the chain is loose, it makes whistling and popping sounds, which increases as the engine speed increases and disappears when the accelerator pedal is released. But if the car has a timing belt, there will be no alarming sounds before it breaks. You just need to respect the changing interval, which is 60.000 km to 100.000 km or 32.000 miles to 62.000 miles.

4. Problems with the pistons and valves

The crankshaft main bearings could be the fault of a malfunction that produces a dull metallic sound when the gas pedal is pressed sharply at idle speed. The frequency of the knocking increases with the speed of the crankshaft. A sharper sound with irregular intervals is caused by excessive crankshaft axial clearance.

Even sharper noises are caused by crank bearings. A slight knock could also be produced by bent pistons.

Exhaust or intake valve knock, which is heard at regular intervals is most likely a signal that the valves need adjustment or there is increased wear of the valve mechanism or one of the components of this mechanism is bad.

If the knocking intensifies when the engine picks up speed, it is most likely due to increased valve clearance.

5. Car’s exhaust system

If the exhaust system is bad, the sounds produced are unique, first the muffler rattles, then there is the sound like a small explosion, which, the sooner you contact the specialists to eliminate this problem, the cheaper the repair cost will be.

There are also other symptoms that are related to sounds and knocks when the car accelerates

Transmission problems

The transmission’s primary shaft bearing is most likely the cause of knocking sounds when the engine is running. These sounds can be louder when you press the gas pedal. This symptom should not be ignored and a thorough check is needed, otherwise, the car will simply stall in the middle of the road and you will need the help of a tow truck.

Bad rear-axle rods. 

During braking and starting rear-wheel-drive cars knocking is often heard as a result of the wear of the bushings. Or after a sudden pressing of the gas pedal, the rear of the car is pulled to the side. In this case, the direction of the car’s movement must be corrected by the steering wheel. Thus it is necessary to replace the bad rear rod.

A bad CV joint

Failing CV or constant velocity joints can cause knocks when turning, especially if you increase the speed in a turn or you turn the steering wheel from side to side at idle. In this case, You need to replace the CV joints fast to avoid costly repairs later.

Bad front axles

Sounds like a knock and a hum can be heard from the car’s axles. If the hum increases when the speed is increased, it means they are faulty. The half axle itself is not broken, but it has a weak point: the bearing. It must be replaced as soon as possible together with the half axle.

Worn shock absorbers

Usually, shock absorbers don’t emit any sounds, however, if sounds are still present, you can hear them well when driving on uneven roads.

The noises are often squeaks and knocks. Their major problems are the wear of rubber bushings and oil leaks. To see if shock absorbers are bad, you need to press hard on the trunk and fenders and listen if there are knocks. If the body continues to balance after you stop pressing the trunk, then, most likely, the bushings and pads are worn out. 

It is dangerous to drive a car with bad shock absorbers because when you press the brakes suddenly, the braking distance increases. In addition, the cause of their fault may be oil leaks from the shock absorber, which is noticeable, because you will see dirt and smudges on those parts. Failing shock absorbers need to be replaced.

Failing ball joints

If the ball joints are bad, they will creak or knock when driving on uneven roads and bumps, and these sounds will increase in intensity and speed if you press the gas pedal. It is necessary to replace them as quickly as possible, otherwise, the car will stand in the middle of the road, and the wheel will be unnaturally turned out.

Bad hub bearings

Front-wheel bearings can make both knocking and other noises. These sounds occur as a result of wear on the bearing rollers and knocking is due to strong play in the bearings. The intensity of these sounds is bigger when you accelerate the car.

It is not difficult to diagnose this issue yourself. To do this, raise the left side of the car with a jack and turn the wheel sharply on both sides. If there is a hum similar to the noise of a metal ball rolling on a metal sheet, the bearing must be replaced. 

Next, grab the top of the wheel with your left hand, and the bottom with your right hand. Now start shaking the wheel back and forth. If it loosens even just a little bit, you should go to a car workshop to replace the hub bearing, to avoid other serious problems later.

Scroll to Top