and checked the rear greases for metalic parts? sorry i do not know what was ment by that.did the mechanic take of the rear rotors to check the e-brakes?has he spun the rear tires for noises...this is sure a wierd noise that u are saying.maybe do this ,take the rear drum of and see if the dust /backing plate is rusted and the rotor iis touching the backing plate...or possible your wheel bearings are starting to pack it in.good luck