I suspect that the cause is either an exhaust leak or some other exhaust problem, or a differential problem.

If you can, raise the car on a garage lift and reproduce the noise (start the engine, engage drive, and bring the speedometer speed up carefully to make the noise again). DANGER: do NOT exceed 100kph on the speedometer - you may over-speed a tire and cause it to come apart at high speed, which can produce deadly flying tire debris. While inspecting the car from underneath, the cause of the problem may become very obvious.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC