So, I've been testing. No popping so far as I can tell, certainly not at low speed or in reverse. Thought I heard a couple at higher speed but could also have been bits of ice on the road.

It seems to be more apparent in long right hand turns. I took a few videos:

YouTube - X5 grinding
YouTube - X5 noise 2

Could also be related to the new rotors/pads but seems less likely.