A low whine or moan

[WBissell]

Hello. I have an issue I hope you all can help me with. There is a low whine or moaning sound coming from, I think, my transmission or t-case. Here's what I know:

1. The noise is present only when the transmission is in DRIVE.
2. The noise goes away when I'm coasting with transmission in NEUTRAL.
3. The noise IS present when the t-case is in NEUTRAL and the transmission is in DRIVE.
4. With the t-case in NEUTRAL, trans in DRIVE, the noise is louder when the truck is moving at a higher speed (say above 30mph) than when the truck is sitting still with the engine rpm's up above idle (don't have a tach but my guess would be around 1500 rpm).
5. The noise DOES vary with engine rpm when t-case is in NEUTRAL, trans is in DRIVE.
6. With t-case in the normal position, the noise seems to get louder with the speed of the truck INSTEAD of engine rpm.

Did I lose everyone? Any ideas?

As background, I just had the transmission rebuilt by DTT back in April. The trans has been performing flawlessly. Bill Kondolay gave me a few ideas to check but thought I'd try you guys as well. I already made sure the cooler lines aren't rubbing by inserting tie-wraps between the lines every few inches and I made sure the clamps are tight.

Thanks in advance for any suggestions or help.

Wade