After about 700 miles on a completely rebuilt rear end, i starting experiencing a high pitched noise from the differential. The noise was only heard from 5-40 mph. When I had it on jack stands, it sounded as if it was coming from the pinion bearing area. I replaced an already new u-joint to confirm that it was not my problem. Anyway, after further examination, it was determined that the dust cap covering the pinion seal was making the noise. This was determined by spraying a small amount of lithium lubricant inbetween the dust cap and the pinion seal. With the lubricant added, the noise went away. However, to prevent the noise from reoccuring, the dust cap was removed. The dust cap is a hard plastic piece that rides against the pinion seal. According to my source, removing the dust cap will not affect the life of the pinion bearing or allow any foreign debris into the rear end. By the way, I used high quality bearings for my rebuild and the dust cap was supplied with the kit. I hope this may help someone and save them time and $$ in identifying this annoying noise.