I had some squealing noise but only when it has been running hot. Yesterday as I had the discs and pad replaced, we noticed the brake pump needed repair as the seals were worn through. NOW, they suspected that could have been the source.
It may be a little different in your case but I would suggest taking out the discs and see if its aligned sturdy in place.
Otherwise, would it be coming from your front wheel assemly?
126 tailed by a 203, 129 leading the pack.