Why does my Audi A2 engine make a knocking noise when I accelerate?

I've noticed that my Audi A2 makes a strange knocking sound whenever I accelerate. It doesn't happen when I just rev the engine while stationary. What could be causing this issue?

The most likely cause of this knocking noise is engine detonation or pre-ignition. This occurs when the fuel mixture in your engine combusts prematurely, causing the knock. It can be caused by a number of factors including low-quality fuel, incorrect ignition timing, or carbon buildup in the cylinders.

It could also be caused by worn or damaged piston rings, which can result in a loss of compression and lead to detonation. A thorough inspection of the engine's internals may be necessary to diagnose and fix this issue.

Another possibility is that the engine is overheating, which can cause the fuel to ignite before it reaches the spark plugs. This can be caused by a faulty cooling system or a clogged radiator. It's important to address this problem before it causes permanent damage to your engine.