Knocking noise

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.