Whineing noise

[Bucmaster]

My wife's 1999 5.0 AWD Explorer has a whine that only changes with MPH change. Applying the brakes has no effect on the noise. Seems to be more prevelent on decelleration. Kinda has a whine on accel and almost sounds like a grinding noise on decel. I searched the forums here and thought it might be the VC in the AWD transfercase, so I changed the tc fluid. Looked like brand new oil. My question is, if the VC is going bad, can the fluid still be new looking?
My next move may be to open up the rear end and check the pinion bearing and the carrier bearings.

 

[imp]

VC is a doubtful source of whine. If you can pinpoint source as from front or rear, my money would be bet on differential noise, as you mentioned. Failing bearings there usually give themselves away before they can allow enough "play" to affect the gear tooth contact pattern, which causes whine due to misalignment. Only a few thousandths of an inch are enough to cause gear noise, but that is usually worst on drive side, under load, rather than coast, where tooth loading is much less.

If you put the vehicle up off the ground, all 4 wheels turnable, run in gear at idle, stethoscope both differential housings. A lot may be determined this way.

If differential-involved, drop driveshaft, turn pinion by hand. Pinion bearing roughness will be pretty evident, Diff. side bearing damage will not be as discernible this way. As an aside, the old Ford 9-inch axle allowed us to remove pinion via it's "carrier", from the front of the pumpkin, without disassembling the whole thing, to replace pinion bearings. Wish I had one of those in every vehicle! imp