My '07 Xterra has 100k miles on it. According to the Carfax report, the previous owner had the differential replaced/overhauled at a Nissan dealer at 70k miles. Ive only had the truck for a few weeks, just recently drove on the highway. Noticed a faint whine from the rear end between 50-70mph. Goes away when I accelerate and decelerate. Had a slight leak by the front pinion seal and my mechanic changed the fluid which was light brown and frothy. He pulled the vent tube and cleaned out and found no sign of grit or chunks. Did some highway driving today, still have slight whine but no leak now. Is the Whine anything to be too concerned with or could this just be a vibration thing? Planning on having him pull the cover off and check the gears for wear and damage. . .

I would have him pull it and check the indexing (mesh) on the gears. A whine like that, that stops with a load change (accell/decell) point to an improper indexing, or "backlash".

Jeff!

Thanks, That actually makes sense then just having to rebuild, especially when there is no sludge or chips in the diff. fluid.