High-Probability Causes:
* Low engine oil: This is the first thing to check. Low oil pressure can cause the engine components to knock against each other. Check your oil level immediately using the dipstick. If it's low, add the correct type and amount of oil specified in your owner's manual. However, if the knocking is already present, adding oil might not solve the underlying problem, and could even worsen it.
* Rod knock (serious): This is a catastrophic failure of a connecting rod bearing. It's characterized by a rhythmic knocking that increases with engine speed. Rod knock requires immediate attention; continued driving will likely result in a complete engine seizure. This is usually accompanied by low oil pressure and metal shavings in the oil.
* Main bearing knock (serious): Similar to rod knock, but involves the bearings that support the crankshaft. This is also a very serious issue requiring immediate attention.
* Piston slap: This is a less severe knocking sound that occurs when pistons are slightly loose in their cylinders. It's often more noticeable when the engine is cold and may lessen as it warms up. Worn piston rings can contribute to this.
* Connecting rod bearing wear: Similar to rod knock but may not be as immediately catastrophic. It is likely to worsen over time if neglected.
* Wrist pin knock: The wrist pin connects the piston to the connecting rod. Wear in this pin can also cause knocking.
Less-Likely (but still possible) Causes:
* Loose or damaged timing chain/belt: While less likely to cause a *knock*, a worn or damaged timing chain can make a rattling or knocking sound, especially during startup.
* Exhaust manifold leak: This can sometimes cause a knocking sound, but is usually accompanied by other symptoms like a loss of power or unusual exhaust fumes.
* Something loose in the engine bay: Though rare, it's possible a loose bolt or other component is striking the engine.
What to do:
1. Check your oil level immediately. If it's low, add the correct type of oil (refer to your owner's manual). Do not drive the car if the knocking is significant or continues after adding oil.
2. Listen carefully to the knock. When does it occur (only at startup, all the time, only under load)? Does the speed of the knock change with engine RPM? This information will help a mechanic diagnose the problem.
3. Don't drive the car extensively if you suspect a serious problem like rod or main bearing knock. This can cause further damage, leading to costly repairs or a complete engine replacement.
4. Take your car to a qualified mechanic as soon as possible for diagnosis and repair. Ignoring a knocking engine can lead to catastrophic and expensive damage.
This information is for guidance only. A proper diagnosis requires a professional inspection. Ignoring engine knocking can lead to significant and costly damage.
