Could be a pinion bearing.

 

It happens when the truck is sitting still in reverse so that should rule out the pinion bearing. After reading the original post over again it almost seems to me like a clutch fan sticking but a noise is so hard to diagnose with text.

 

Yup good call, gotta run with the fan as well.

 

Update to my original post...

About a week later, the transmission began to mis-shift. Dropped the pan and found metal shavings. Took it to the trans shop and they said the Torque converter went bad and sprinkled metal through the transmission and clogged the solenoids. A transmission rebuild fixed the problem.

 

Thanks for posting your solution. It will help someone else diagnose their problems.

 

I stumbled on this thread, and it sounds exactly like the issue I described in my own thread, but none of the suggestions proved fruitful. I certainly hope my torque converter doesn't explode. What even causes a torque converter to go bad?

 

Sorry no answers. I have the same whinning sound. Mine starts when I first go, then it slowly stops !!!