My car started doing this last month and book it in with the dealership and they changed the whole rear brakes (Disc, pads and hubs) under warrenty. So far no sound now, fingers crossed.
Also mite want to check out these links:
https://www.babybmw.net/threads/loud.../#post-1736349
https://www.babybmw.net/threads/reve...4#post-1736336