It was definitely not the fan. It was making a sort of oscilating wine noise that was most prominent at idle, no matter how warmed up the truck was. As soon as we took the belt off the noise stopped and has not returned after we reinstalled it.

Strange but the best I can figure (as Bogatyr suggested) is that the belt tension was off. It doesn't seem worth it to pursue getting the suspect pully's replaced since they no longer make noise, but if it comes back I definitely will.

I just wish the dealer would have taken a look at it before I picked it up instead of just saying it was the SC and the noise was normal.