I had similar symptoms just this week with my 300TDT with a 4 speed out of a 240D (460K miles). I thought of all the same things as you did with the driveline ect. Turns out the transmission gave out completely. I think the ticking was broken parts in the gear box. When I drained trany oil, there were some pretty big chunks. Now I am on the market for a used 4 speed. It is a first for me, seeing a manual fail like this. kinda strange. I wish you the best.

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1980 300TD with 1985 300TD Turbo motor and 240D 4 speed.