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