With a squeal like you have there will generally be rubber around the pulley that is slipping. Which belt is making noise?

The only one! Serpentine belt on the 3.5. I guess my tensioner really is bad...there goes another $150.

The belt I took off looked like new, just was a little shiny on the ribbed side. No cracking or shredding at all. No rubber residue on any pulley.

Take a hand full of fine dirt and dust the belt while it is running....Try it I dare you...

on the 3l, I've found that not all alternators are the same,
and they don't always tighten down the same, either.

What happened to mine was that the alternator sat just a touch
forward- not much, but just enough to push the belt into
the edge of one of the idlers. I could at one point
actually pry on the alternator while it was running
to make the squeak go away. I put a thin washer
in one side of the mount to shift the alt just a touch-
problem solved.

So look at the ledges on the non- ribbed pulleys-
you might find one is shiny where the belt's been touching it.

That might help find the culprit...

hth

t