I am going to go for a tensioner bearing having failed. Look on the passenger side of the engine at the serpentine belt and check to see if you can get any more than about a 1/4" of movement pushing in on the belt.
Check to see if you can move any of the pulleys or the tensioner by hand, in a "wobble" fashion (technical ain't I?)
You shouldn't HAVE to, but you may need to remove the belt (3/8" breaker in the tensioner square) to be able to determine if a pulley or tensioner is actually loose.
replacement of the tensioner can be done on the car, with little to no removal of other things. It is literally as simple as removing the belt, then undoing the two bolts and removing, reverse to install.
