Originally Posted by doyll
It's probably a glazed belt caused by not being tight. Tightening it now might stop it, or you might get it too tight and burn out pully bearings.
Or like I did back in the day with my Isuzu Rodeo, overtighten it to the point that while driving down the road the belt exploded under the hood leaving me with no A/C, and belt shrapnel under everywhere under the hood. I kept one part of it, it kinda did the same thing to the belt that happens when a tree gets struck by lightning, it like split down the ribs and blew apart.
Anyway, it might be your compressor, usually when things along the belt are going out, they get hard to turn, and when that part is engaged, it can cause it to slip even with good tension on the belt.