I have a 1990 520i E34 that clicks under heavy braking from the pass side rear i think. It also doesnt seem to sit level, the driver side at the rear is a few mill lower than the other.
i had the same problem although with a 2004 320d, if i was pulling into the driveway i would here the click you described as i was braking, i also noticed that one side was sitting lower, i jacked car up and found that rear shock was leaking. replacing the shocks solved the problem.