My 1985 300D turbo diesel makes a light thump thump thump when backing up. Any suggestions on where I should look?

Check the driveshaft bearing support mounts......

Thanks. Will do.

IF the noise happens ONLY in reverse then you could have WORN reverse clutches allowing the reverse piston to HIT against the K1 clutch drum..

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Only happens in reverse and not all the time.

The Beginning stages of the K1 drum rubbing against the B3 piston, is a massive delay, chatter into reverse. If you don’t have the symptoms, it’s not the K1 drum, check the center bearing and support, check the drive shaft for any rub marks, loose muffler or bracket , broken or loose transmission, differential mounts.