What could cause a rear end to whine after sitting for a while?
 

So my buddy parted out his 02 Z28 and gave me the complete rear axle. It had around 40k miles on it and was pristine. I swapped it in my car and it whines. A lot. Really loudly. You can start to hear it even at 20mph, and by highway speeds it's brutal, louder than the stereo. Mostly it is under any kind of load, while coasting it's maybe 10% as loud, almost unnoticeable. No problems with turning or anything.

I changed out the fluid, it looked great. No change in sound. Prior to him giving me the rear he said it didn't make any noise.

Any thoughts on what could possibly cause the whining? The car sat (with fluid) 2-3 years before I swapped it into my car. I'm thinking even if there were some corrosion internally that it would have worn off pretty fast. Though maybe pitting on the gears could be the cause? I've driven the car around 100 miles with no change in sound at this point.

Just wondering if there's any possibility of salvaging this nice rear or possibilities of things I could have done wrong during the swap that would have caused the sound. I figure someone would have had to have run into this at some point.

Dope

Did he do many hard launches/60 foots with sticky drag radials during those 40K miles???

Not particularly. Any racing was generally done on the road course/HPDEs. As mentioned, it did not whine in his car prior to parting it out. Very strange. I was thinking maybe pinion angle could cause some noise but hell, I'm still running the stock TA. How far off could it be?

Dope

Doubtful pinion angle could cause a whine, it causes vibration. My hunch is that the top of the ring gear was out of the oil long enough to rust and now its pitted and causing the whine.

Pinion angle is a bad vibration not a whine. I had a bad gear from Motive one time that the metal was flaking off on some of the teeth edges and it was ridiculous how loud it was. The flakes got in the bearings and ruined them which is what was whining so loud.