Had the same thing happen to me on my '86 535. Check the Guibo/Flex disc at the transmission output flange- driveshaft connection point. BMW used some disc's with an external steel balancing weight bonded to the rubber disc and over time the bond breaks down and the ring comes off. It will spin on the output flange area and bang against the floorboard area and the shift linkage causing all kinds of noise. Get under and cut it off with a hacksaw. It will run fine without the ring. Order a new Guibo disc from an aftermarket supplier and install it later. You may want to check the mounting bolts to your shift linkage while under there as they have a tendency to work loose and cause hard shifting problems. I just repaired my wife's '85 635 for the same problem.