I have the same sound. Im in the process of compiling data to post here.

My clicking/ticking is louder in the car than it is outside of it. Its distinct inside the cabin. I just got an automotive stethoscope yesterday and it will prove very useful. In a pinch, try listening with a long screwdriver with the plastic end against your ear

First - listen to the vacuum pump and timing chain to rule them out. Then listen to all of the injectors. I have done this and I can hear that my sound is not coming from these places, it seems like its coming from the rear of the engine towards the back or even the torque converter.

Im interested to see what other folks chip in with. Try listening for the things I mentioned earlier and report back...

dd

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car