Has anyone had any problems with their air conditioner pulleys and
the serpentine belt?

I just found a problem with my 99 Explorer, and came rushing to this group
for help.

Basically, today I noticed a strong clicking coming from under the hood.
I had turned on my car and was still parked with the AC on. I popped
the hood and found that the pulley connected to the AC was alternating
regularly between spinning and not spinning. If I turned off the AC, the
pulley stopped spinning completely, of course with the belt still going
over it.

To tell you the truth, I don't know much about cars, much less the
specifics of AC units, and I don't remember what exactly that pulley was
doing before, but this just doesn't seem right. The serpentine
belt going over a locked pulley just can't be good for the belt, can it?

I'm about to go on a ~9 hour drive through the desert too, so I'm going to
want to take this to get fixed ASAP, but any info you could give me would
be awesome. It would be great not to seem like a moron when I go up to a
mechanic.

The reason I ask about Explorers in general is because I seem to recall my
parent's had a very similar problem a couple of years ago.

Thanks