I just changed both belts on my 96, with AC. (I didn't loosen the pulley on the PS belt, so that didn't change.) With the new belts on and the car still on a jack, I ran it and it sounded fine. But then when I started driving, I got this mechanical sounding rattle. I loosened and tightened the tension on the AC/alt belt, but it didn't seem to make much difference.

It's definitiely not the typical belt squeal. Video here: http://www.youtube.com/watch?v=cIeAAsZAxGE Does this sound like the idler pulley is shot?

 

yea, but do this get a screwdriver and put it to the adjustment spot on top and your ear to the other end to see if thats where it's coming from. sometimes changing the tension will shift the weight on the pulley and cause it to wear in a new spot. this goes for any pulley... so it's possible its your a/c alt bearing but prolly not.

 

That is a belt noise? I would suggest removing the belts and starting engine again and see if noise exists. Also, I would check to see if your crank pulley is wobbling.

 

All is well. My wheelwell skirt is a little bit trashed, and it turns out a loose end was rubbing against the belt by the AC condenser. A few snips, and it's fixed. Thanks for chiming in guys.