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My April 1998 build date Carrera 2 has been having a cold start knock since I bought it in 2015, The noise would appear a few seconds after startup, and only at idle. It would go away after a few blocks but would often reappear after a few hours of the car sitting. Today while I had the car on a lift I noticed the coolant pipe in front of the sump plate moved in its clamp and when it did it made a noise similar to my knocking noise. The clamps along the side of the motor did not move. Sadly these clamps are welded shut so putting a rubber shim between them and the pipe is impossible. I sprayed with Plasti Dip on the coolant hose and let it cure up some before sliding back in place. Too soon to tell if its been cured but so far its been silent.
Picture is of the clamp pushed aside from where it was clamped you can see the marks from the vibration.