Everyone seems to have ignored my previous post with photo (p. 2 of this thread). If you would ask your tech to clean and lube this spring, I think you will have found your problem. It has been a demonstrated fix in at least three M3 cars that my tech has inspected for this noise. You can push and bend on your disc shield forever, but that is not the problem (unless, of course you actually have a stone or something in there, which can happen).
2011 Space Gray Coupe, DCT, ZCP, StopTech ST-60/ST-40 BBK.