Bearing-like Noise that has caused me to change transmission 3 times

"When you raise the RPM does the pitch of the noise increase? Also what lead the mechanic to believe it was the transmission? Are there any drivability problems? Does the sound seem to be coming from that part of the truck or more towards the front?

To me that sounds very similar to a pulley bearing. Get a mechanic’s stethoscope and see if you can narrow down where the sound seems to be coming from."

KISS- Keep It Simple Stupid. So let's back up and slowwwwww down. Now this is just basic checks to eliminate possibles. You have a bearing going out you can hear it till it starts heating up then when it's as hot as it can get it quiets down. You can run it till the noise starts or check it cold. If you run it till the noise starts you can check it by touching the center of your pulleys to see if one is getting hot. CAUTION!!! Blistering your fingers could result. But I suggest while it's cold take the serpentine belt off and spin and wiggle all the pulleys by hand except the crank (biggest pulley bottom center). Even cold you should be able to feel movement(wiggle) and hear(spinning by hand) noise from the problem pulley. To guess I would say it sounds like the lower idler pulley. Is your a.c. pump on the bottom passenger side of the motor? Dose the sound change when you turn the a.c. on and off?