Transmission mount gone soft will cause drumming on accleration. I'd check it, hopefully this is the case.
The 603 3.5L is known for bending rods, but if that is the case you will have a terrible clank and horrendous oil consumption with copious blue smoke.
A dull thump would make me think of the tranny mount, or possibly the center bearing mount going -- if that is the case, the floor pan will vibrate at the rear of the front seat. If you put you hand on the drive tunnel when it's making the noise and you feel it vibrating, it's the center bearing mount.
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!