An automotive stethiscope would come in handy to find this. Another old school trick is get a really big screw driver and place the handle to your ear while touch the metal end in suspect areas. when you find the source it will be very loud in your ear. This is also an easy way to find a worn pulley bushing. It does sound a little like the clucted s/c pulley but you also have one if these on the a/c compressor as well.