My brakes do exactly the same and it is a bit unnerving. However it doesn't just happen once: if I am moving at very slow speed and have to break (your usual traffic jam), I get a thud and feedback in the pedal (that happens very rarely). I was reading about the issues that my 300E initially came with and found that there was some problem with ABS (if braking at low speeds, the squeal would activate it,
consumerguide.com).
Any ideas?
Thanks.