Squeal sound form Trans or Transfer Case

2001 GMC Sierra 1500 HD
When I start from a stop I am getting a squealing sound, like a vacuum leak or loose belt type sound, from the transmition or transfer case area.
The sound doesn't change with speed but if I let off of the throttle and slowly give it gas the sound will go away until the next stop. s
Sometimes it will squeal at a cruising speed but the sound is very low. The sound doesn't happen every day just every now and then.

Any other questions that might help you with the diognosis let me know.

Thanks,
Marc

you want to check the out put bearing of the transfer case, i had a 98 tahoe the bearing had a plastic retainer to space the balls. The retainer pops off and lets the ball all to fall to the bottom.you could be hearing the bearing spinning in the allumanum?? case. go to the GM dealer ask to see that bearing if it has the plastic retainer. if does i bet that is your problem. hope this helps.