Interesting, I have had something similar with my fronts - it is on about 66k miles but they are the originals, so the discs should not have ever been off for anything to get between them and the hubs.
It is a funny one because it is very noticeable from about 65mph upwards but absolutely nothing below that. In warmer weather and if I give the brakes a real pasting - such as a track pad break in procedure - it seems a bit better. And when the rear discs and pads were changed this summer, it went completely, only to return after a few thousand miles. That was really odd.
It is an auto, but I try not to sit on the brakes after any remotely hard braking. I keep meaning to get the front pads/discs changed to see if it sorts it, but BMW's costs are high and the fact it is only at high speeds makes me vacillate - especially when plenty of people on here have done this and not seen a benefit! It seems a common BMW thing - maybe it is made worse by the soft bushes designed for RFT...