My experience is with mid 80's vintage hi-mileage 5 series. Your new car should not have worn bushings such as I am experiencing.
I just yesterday replaced the RH rear carrier bushing on an 86 535i which had a loud clunking coming from under the RH rear seat. This bushing allows the entire subframe to float about 1 inch vertically. It would clunk (actually more of a thump, not a metallic sound) when taking-off with a little more than light power applied. In other words, any aggresive driving at all and it would thump badly on accel or decel or even when using reverse.
To help troubleshoot, try to determine if you can localize it to LH, RH or center and if it is felt under the seats in back. Have a mechanic or friend sit in back and listen/feel for where it's coming from.
Cold be driveshaft, differential, CV joints=clunking or metallic sound
Could be any one of a bunch of rubber bushings=more of a thumping sound
