On my 300sd, I am getting a very obnoxious,intermittent cicking noise coming from the area where the transmission meets the engine.I verified this by using a stethoscope. It will usually happen while idling in drive, but can happen anytime i am idling. The sound goes away when under power. I swapped the engine and transmission a few months ago, and have not managed to get rid of the noise. I have a spare tranny flex plate, and torque converter I can swap in if necessary.
Do you guys have any thoughts on what could be causing the clicking?

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1979 240D with a 617 turbo diesel.