It sounds like either you aux belt is catching or one of the bearings in the aux belt path is on the way out. If you can get hold of an automotive stethoscope use it to try to track down exactly which point is making the noise. They are not very expensive but if you can not find one using a long screwdriver or other rod sometimes works as well move it around whilst placing the other end against you ear. Watch out that you don't catch it on the belt.
