Most Common Causes:
* Low Engine Oil: This is the easiest and cheapest thing to check first. Low oil pressure allows engine components to rub against each other, creating a knocking sound. Check your oil level immediately.
* Worn Connecting Rod Bearings: This is a serious problem. The bearings connecting the pistons to the crankshaft wear down, leading to a knocking sound that worsens with engine speed. This often requires engine rebuilding or replacement.
* Worn Piston Rings: Worn piston rings allow combustion pressure to escape into the crankcase, leading to a knocking or rattling sound. This can also contribute to low compression.
* Damaged or Failing Piston: A damaged or cracked piston can cause a loud knocking noise.
* Rod Knock: This is a severe issue where the connecting rod bearings are failing. It's usually a very distinct, metallic knocking that intensifies with engine speed. This requires immediate attention to prevent catastrophic engine failure.
* Valvetrain Issues: Worn rocker arms, lifters (hydraulic or solid), or pushrods can also cause knocking, particularly a ticking or tapping sound that may change with engine speed.
Less Common but Possible Causes:
* Loose or Broken Timing Chain/Belt: While usually more of a rattling or clicking, a severely worn or broken timing chain can lead to knocking sounds.
* Exhaust Manifold Leaks: While usually more of a ticking or rattling, significant leaks can sometimes sound like a knock.
* Loose Engine Mounts: While unlikely to cause a true *knock*, loose engine mounts can allow the engine to move excessively, creating sounds that might be misinterpreted as a knock.
How to Diagnose:
Pinpointing the source of the knock requires a mechanic's expertise. However, you can gather some information yourself:
* Listen carefully: Does the knock change with engine speed? Does it change when accelerating or decelerating? A knock that gets worse with higher RPMs usually points to internal engine problems.
* Check the oil: Low oil is a quick and easy check.
* Check the oil pressure: Use an oil pressure gauge (available at most auto parts stores) to check your oil pressure. Low oil pressure points to a serious problem.
Important Note: Ignoring a knocking sound can lead to catastrophic engine failure, resulting in expensive repairs or even a need for a new engine. If you hear a knock, have your vehicle inspected by a qualified mechanic as soon as possible. Driving with a severe knock can cause further damage and potentially lead to a dangerous situation.
