Probably some wear, which can cause a whine.
The other thing that can happen with closed drive lines is that the seal at the top of the torque tube behind the transmission can go bad and the oil from the transmission can leak down the tube to the rear axle housing. The transmission is then dry and the brass clutches in the transmission start to breakdown. You'll know this when you see small brass particles in the bottom of the transmission when you drain it. This, of course, can be stopped with an open drive line and leads me to the reason I dumped the torque tube concept. I rebuilt my stock transmission, as stated, and after I reinstalled it, I was constantly checking to be sure the oil form the transmission wasn't draining down the torque tube to the rear axle. It became an obsession and my greatest fear was that I would be on a road trip and loose the whole transmission-real axle in the truck due to it "burning up".
On your other post, you mention trying to keep the truck a "period correct" push truck.....I think you can do that easily with an open drive line a T-5 transmission. Take a look at my pictures and you'll see very little that would give you a clue that my truck isn't a basic stock truck.
