Just curious....what would be the most likely cause of a rear end clunk when upshifting from 3rd to 4th? Only does it when it is cold. When all the fluid warms up not only does the transmission shift smoother, but the rear end clunk is gone as well. I also noticed if you rev the motor in park when it is cold, you will get the similar clunk when you quickly lift off the throttle.

This is on my brother's 83 300D by the way, not mine. Checked the flex discs and the are fine. I'm thinking mounts or exhaust system moving, or???

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)

Sounds like you are low on differential fluid or it needs to be changed. That does not explain the clunk when in park, though. How are the transmission and differential mounts? Exhaust could banging around too but I don't see how that would change with a hot or cold engine.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge

I'm thinking a mount as well. We changed the diff fluid in the spring. I'll get him to check it. Interested to hear if there are more ideas as well

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)