Check the flex disks, differential mount, and vacuum system. An excessively hard 2-1 downshift will make the rear end thump.

Also check the differential bushings -- they can be bad, as can the rear carrier bushings. Anything loose will cause a thump.

You could also have a loose exhaust that thumps when you floor it, too -- check the rubber hangers and the condition. Not a typical problem.

Peter

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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!