That's a major bummer. My rotors and pads were replaced a few weeks ago, but with all the snow, I haven't driven it too much. But the noise has not come back yet. Probably only 150 miles since the service. Tell BMW to put Brembo brakes on it if they can't fix the issue.
It must not affect all the M5'a though, or this board would be lit up.
Good luck with a fix.