Check the disks. I've known crusty outside edges on old worn disks cause some interesting noises. Also check the inward facing side for surface rust as
@Burnerman has already suggested, which might indicate a sticking pad or piston - as problems are often hidden by the stone guards. On the subject of stone guards, make sure there is nothing lodged behind them and make sure they are secure. They are thin metal, get hit with stones as the name suggests, and have a nice habit of rotting and disintegrating on VW group cars, where they can make contact with the disk or even jam against it.