Squealing noise past 2000 rpms

I have a strange noise coming out of my blazer. :banghead: It happens when I'm under load past 2000 rpms. The sound is a very noticable squeal. The sound spools up in pitch with the engine. While going a long if I shift it in to neutral I can rev it up and make the sound also. That noise is very noticable from the inside and outside of the truck. It is not the brakes, nore the belt.
What could this be? :confused: Please help!