When I shift into any gear I hear a thumping noise in the rear of the vehicle. on 2000 BMW 323Ci

Anytime I shift I am getting this thumping noise. When the car is idle and I move the gear shifter I can hear this thump like something is loose. The car is a 5 speed manual transmission. This thumping sound only happens when I shift.

Asked by for the 2000 BMW 323Ci
could be one of a number of things. driveshaft flex disc?
rear subframe mounts? car needs to go up on a lift and a
visual inspection by an experienced BMW technician. whenever someone says "like something is loose", have it
inspected NOW, before it falls off!
Thanks for the advice!