When it gets hot (after driving 15 or so miles), and I let off the gas to coast, or start to brake, the car emits this strange fluttery sound from the center or rear of car (think zip tie in bycicle wheel) ... and it speeds up in its cycle the faster I slow down (more fluttering if I'm jamming the brake as opposed to coasting to a stop). Does it with the clutch in or out, regardless. I don't really even know where to start since it only does it hot.

When I apply throttle it goes away.

Help.

Car is a 6/89 (MY90) 535im w/ 141k on the clock.