Front flex disk is a 60,000 mile part most of the time, causes the thump.
Another source of thumps is the differential mounts on the rear subframe. The rubber deteriorates, and the steel sleeve bangs on the subframe. You can check by taking a peek at them from the rear of the car -- if they are not both centered, or if they more when you apply upward force with a floor jack, they are toast.
If you are also getting some torque steer, pull the rear wheels and take a look at the torque link -- its the diagonal one from the wheel carrier to the inside of the subframe. Shreds of rubber indicate a bad bushing, meaning a new link.
The rear subframe mounts will eventually go bad, too, will also cause a thump.
I know all this because I just put some links in the 300D, and will be doing the rest (might as well put them all in, why do it one at a time?) and subframe mounts, mine are cracked.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!