I have an 08 Sport Trac Adrenaline. Fantastic truck!
Lately I have been hearing a whining noise from either the transmission or rear end. The tone varies according to speed. Most noticeable when it's cold.
I thought it could be rear wheel bearings but I had those checked.
The mechanics I talked to are stumped. Or maybe just dumb.
Any suggestions on what I should be looking for? All fluid levels are correct.
It has about 170,000 km /105,000 miles on it

 

Carrier bearings inside the diff are suspect. Pinion bearing is suspect.

Pinion whines at driveshaft speed, probably 3.73 times wheel speed. (whatever your gear ratio is)

You might be able to tell which it is by how fast it builds as you speed up.

U-joints checked?

 

Rear end/pinion bearing noise is a pretty common complaint on the Explorer and its variants. Often the sound starts becoming audible around 50-55 mph and may change/go away under acceleration.

At one point Ford changed the recommended gear oil weight in an attempt to quiet the whine, but the results for owner's on this forum anyway have been mixed.

-Rod