So my '82 633csi recently started up with a sort of knocking or clicking sound coming from the rear. It is quite difficult to describe what it sounds like, but nothing big and harsh but just a loud and percussive almost metallic tapping sound from the rear of the car, almost like a parking brake twanging when backing up in the morning. The sound is external as you can here it from outside the vehicle, but also very loud on the interior of the car. This sound occurs mainly and most definitively when coming off the accelerator or depressing the clutch, anytime that power is being but to the rear wheels and then removed. The sound occurs in a short series of three or four 'knocks' but is quite loud and quite annoying. There does not seem to be any mechanical affects that can be felt like something in the brakes, steering, power delivery, etc. and is merely audible at this point. Previously during the same conditions there was a slight squeaking sound but this has since taken its place, not sure if there is any relation. I've looked underneath but can't visibly see anything but then again I'm not entirely sure what I'm looking for; I've also tried moving with the parking brake 2 notches up to see if it may need readjusting but it made no difference. If anybody has any suggestions on what to check for that would be great!