I've got this buzzing noise that is progessively getting worse. I removed the serpentine belt, started the engine and the noise went away. It almost sounds like the power steering but it doesn't get worse when the wheel is turned right/left. I know something like this is hard to diagnose, but thought maybe one of the belt-driven components is known to go bad. Truck has about 120k miles.
Maybe a bearing going bad?

 

With the belt off and the engine off, spin each pulley by hand, listen for noise and feel for roughness.

If you have the original battery, I'd bet that it might be alternator whine. You might want to go get the battery and alternator load tested - this is a free service at many parts stores in the parking lot, no removal required.