i think your issue is same with me .
noise was only when i backup on garage and now low speed backup making noise at outside too...
i had around same miles with yours (17k) and dealer replaced pads by warranty as i complainted few times in two month ago...
but, issue didn't fix .... I'm gonna go dealer next week to get fix again. i will give you update.
yes bmw said same thing to me " high performance semi-metallic pads " making noise sometime... (( but, noise is coming everyday now when i backup ))
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