2001 F-150 4x4 5-speed. When I first go to take off in reverse, I get some strange noises. Sometimes it's like a harmonic vibration / metallic noise, combined with a light popping, and sometimes I get a fairly high-pitched metallic "squeek-squeek-squeek". It only does this for the first 6 - 8 feet or so, then nothing. I have pulled onto a steep hill and let it roll backwards and it doesn't make any noise at all. I have also tried holding the brake on while attempting to reverse, and it doesn't make the noise. It won't do it in forward gears either, only in reverse for the first few feet, like just before I have time to get all the way off the clutch. It does it regardless of clutch position though, as far as I can tell.

Over the last two weekends, I have tried to get rid of the noise by replacing my tranny fluid with Royal Purple MaxATF, as well as the transfer case. I then did the front differential with Royal Purple Max-Gear 75-90, and rear with 75-145. I also replaced the two rear driveshaft u-joints with AC Delco Super Strength greasables. The sound persists. HELP!!!

 

Could it possibly be the rear brake pads? I know they are about due for replacing. (48K mi. on the stockers) It sort of sounds like it could be the feeler on the rotor making the squeek, and the slack in the pad / caliper making the harmonic vibration...???