When I start my 380SL (85 560 engine) in the morning or anytime after the engine is cold I hear a rapid knocking noise almost like a rod knock. It is not to loud but loud enough that I can hear it inside the car. The car idles at about 1000 rpm until warmup and then drops to 650 rpms. As soon as the rpms drop the knock goes away. I have tried several times to locate the noise before the rpm drop but cannot seem to get it. Any ideas would be greatly appreciated.

After the engine is at operating temp does it still knock when you increase the RPM to 1000? Or does it just knock when cold? If only when cold it could be your chain tensionor and the chain is slack, how is your oil pressure? What is the condition of your oil? History?

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Brian W. Heitman

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Only knocks when it is cold, increase in rpms does not bring the knock back. Oil pressure is good. Oil changed recently. This has been happening for a couple of weeks. Can you elaborate a bit on the chain tensioner?

Thanks for the answer.

Yes - I have the same scary problem. Engine was rattling a bit for about 6 weeks - the best I figured from this website was an engine shock problem. Never fixed it but today the noise is decidely louder - and like a distinct knocking sound that i can hear inside the cabin. Scared that a rod about to go out or a valve coming undone?? Car sounds like a diesel or a Mack truck!
Engine Temp is 80-82, there seems to be some loss of power.
Checked the belt - which seems OK but I smell burning rubber? but then again I am neither an expert on smells (olfactory) nor on W202 engine.
I am looking for relevant codes to check/confirm the engine's health - and guidance/pictures on getting at the belt tensioner (?) problem - this is what the forum wisdom seems to suggest.
Till then - the C280 looks 'purty' just parked there.

Any updates on your engine knocks? Is the car still broken?