2006 3.5l Colorado. Replaced a squeaky serpentine belt and a new sound emerged. Here's a video:
The squeak sounds awfully dry/bad — but when I started it up this morning (about 15F colder than when I filmed) the sound didn't appear.
Thinking it might be a pulley bearing going bad?
Belt tension felt OK I think - was able to turn it about 70-80 deg. Seems aligned on pulleys fine. But I did notice the tensioner pulley had a slight bounce when I first installed the new serpentine belt... don't notice it as much now.