What does it mean when an engine ‘knocks’

While engines can be noisy, often it is the knocking that sounds an alarm to most drivers. Besides indicating an impending failure, engine knocks can also mean entirely different trouble. Your engine might require a complete rebuild or in some cases, a minor repair is all that’s needed to fix the problem. Whichever the case, any strange noise is an indication that your car needs some attention from a mechanic.

In this post, we walk you through the different types of engine knocks, causes, and feasible solutions for knocking among other insights about engine maintenance. If you have any questions about this, please reach out to the staff at Motors On Wheels and we’ll be more than happy to help you diagnose the problem.

Types of engine knocks and noises

Single loud knock

This kind of engine knock occurs once and feels as if something just blew up under the hood. It could indicate increased compression in the combustion chamber, leading to sudden failure of one or several pistons.

Crankshaft knock

Characterized by chattering or clicking noise, this type of knock occurs when you’re pressing the accelerator a bit too hard. Usually, the noise disappears immediately your car gains speed or when you ease off the pressure on the pedal. In most cases, a damaged hydraulic valve lifter or excessive valve clearance causes this noise.

Detonation knock

This type of knock begins with a metallic ping which is then followed by a detonating sound in the engine. Usually, it causes the car to come to a sudden halt as it affects the ignition timing, which is responsible for keeping the engine alive.

Ringing knock

With these types of engine knocks, you hear a repetitive ringing or tapping tone when you ignite the car or when it’s in the idle mode. This knock usually indicates a broken or loose piston ring and sometimes worn out cylinder walls.

Light pounding knock

This knock occurs when the engine is at an even rpm, i.e., when the car is neither accelerating nor decelerating. It might indicate a misalignment of the connecting rod, low oil level, or worn bearing or crankpin.

Slapping knock

With this knock, you will hear a hollow, muffled, or slapping sound inside the combustion chamber of the engine. When it persists after the engine warms up, it points to worn pistons and cylinder walls. Sometimes it could also be due to excessive piston-to-wall clearance.

Common causes of engine knocking

Engine knocking can result from a one-time problem or one that’s developing gradually. Perhaps, you’ve skipped a regular checkup or the engine is finally approaching its end of service.

Below are other common causes of engine knocking:

Defective spark plugs

Spark plugs are small devices found inside your car engine and whose role is to ignite the fuel/air mixture inside the combustion chamber by delivering electric shocks. This is how fuel keeps burning to generate power that drives your vehicle.

Spark plugs, like all other engine components, wear and tear with age and usage. In fact, most car manufacturers recommend changing them after 30,000 miles, while we also advise you to check them when buying a used car.

Defective spark plugs cause incomplete fuel combustion and consequently a drop in engine power. Notably, when you’re accelerating hard, your car might emit black smoke from the exhauster. After a while, a crankshaft knock happens and the noise gets louder with more acceleration.

Low-octane fuel

One thing you must always be cautious about is the quality of fuel that you feed your engine. Fuel generates the power that sustains the life of your engine while also determining how well or poorly it performs.

Knocking that results from using the wrong choice of fuel is common in high-pressure engines, which must compress fuel/air mixture longer before ignition. For maximum efficiency, these engines require fuel with high levels of octane to prevent premature combustion. Although high-octane fuel is costlier than regular fuel, it’s worth purchasing if you value your engine.

You can minimize the chances of knocking due to low-octane fuels by using an octane booster. But this should only occur when you cannot find fuel pumps with high-octane fuel. Further, stick to gas stations that you trust and those known to sell high-quality fuel.

Carbon deposits

In Texas, fuel distributors must include carbon cleaning detergents in gasoline to dissolve deposits that potentially clog up engine cylinders. But these products are not always 100% foolproof and over time, the deposits will still form.

The carbon deposits leave less room for combustion to occur efficiently inside your engine. In turn, this increases the pressure inside the cylinders and can sometimes lead to nasty knocking sounds. The effect is usually lower gas mileage or overheating when oil levels are suboptimal.

We covered this and more areas recently in our post on tell-tale signs that your car needs servicing. Be sure to check it out for more insights.

Worn engine components

An engine with many years of service will undoubtedly be prone to knocking from worn components. Damage to the following parts could cause serious engine issues:


Causes light knocking and is best fixed by performing a cylinder balance test to identify ailing connecting rods.


Causes metallic-sounding double knock and is fixed by replacing worn bushing or tightening the piston pin.


Causes loud knock when the engine is under load and can be fixed by replacing rod or thrust bearings.

Implement these practical solutions for engine knocking

  • Use high-octane fuel or add octane boosters to low-octane fuels.
  • Use engine cleaning additives to minimize deposit buildups
  • Avoid missing regular checkups as advised by the manufacturer
  • If your engine knocking is persistent or is too expensive to fix, replace it with a new one or even consider purchasing another clean car at an unbeatable price.