This may be something you run into if you own a 2.5L powered Subaru. For a number of years, Fuji Heavy Industries decided to use a plastic idler pulley for the ac belt. This will seem all nice and fine for a good while, but the minute is begins to emit the tiniest chirp you'd better be shopping for a replacement part. I didn't know about this little gem of a part and it's propensity for failing in spectacular fashion, so I just oiled it and let it ride. Don't do that!
A replacement pulley that is actually made of metal can be purchased at just about any parts store. If you have a Saab 9-2x Linear though, it won't show that it fits, so just search as if you own a 2004 Forester with the 2.5L. The pulley from AutoZone is Duralast #231029 for $39, and the belt is Duralast #353K4 for about $9.
If you put this off long enough and the pulley fails like mine did, you will also need the Dorman #917-124 Idler Pulley Adjuster Bolt Kit for under $30 with shipping. Order this part from Amazon as soon as you see that it has failed/melted to everything. Most parts stores won't have it and won't know how to order it. The plastic melts to the parts in the kit and just generally makes a mess of things. In my case, the belt ate into other parts after the pulley failed. I consider myself lucky though, because sometimes this pulley fails and pushes back into the timing belt cover, clips the belt, and ruins the engine.