03 GMC Sierra Heavy Duty 1/2 ton Quadrasteer - Had GM mechanics check it out and check rear end for oil level. Said everything looks OK but did find some rust on inside of wheel drums and tried to clean but still have noise. When mechanic power washed the rear end leaf springs, it did seem to quiet the noise until it dried out. Anybody have any ideas what is making this noise. Sounds like a rubbing noise, but only when backing up and sometimes when just starting out going forward.