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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.

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The noise you are hearing is the rear pads shifting position as they grab the disks as they rotate in the opposite direction, as opposed to their mainly used forward direction. A little play between the pins and caliper sliders is normal and this sometimes does click when the brakes are applied travelling in the opposite direction on first application.. Its most likely that the brakes sliding mechanisms are dry and this is why the clicking occurs. It's normally nothing to worry about.

If you try reversing for say 20 feet and applying the brakes a few times in that distance, you'll likely only hear the noise on the first application, subsequent applications should be noise free unless there is a problem.

Just to be on the safe side though I'd recommend a mechanic take a look at the rear brakes just to check them over to make sure nothing is wrong. And at the same time the sliders and pins can be lubed which will likely cure your knocking noise.

I very much doubt that this issue will be as a result of your very light bump.

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