I've got a bit of a strange issue going on with my rear brakes (noise is clearly only coming from the rear; I think both sides but not entirely sure) on my E46 BMW 320d from 2001. When I reverse and then brake, it will often (not always) really loudly click -- it really sounds like something shifting -- and then squeal to a stop. Does anyone have an idea of what this may be?
Potentially related: someone rear-ended me a couple of weeks ago. It was a very soft 'hit', the guy just hooked onto my trailer hook without even damaging the plastic bumper; I had in fact barely felt it (I thought the engine had stalled at first). It seems very unlikely that a light hit like this has anything to do with these problems, but I thought I'd mention it anyway just in case.
Just to be clear: the pads are not worn at all yet. The same sound is not at all audible when braking while driving forwards, even at the same low speeds.