NOT AN EXACT SCIENCE. Its either the belt, the pully , or the spindle.. can you tell us how often it happens? It it when you engage the pto? Is it continuous? Does it ever go away?

The bearings in the pulley on the discharge side are typically prone to the most wear due to the fact that its taller (dual pulley) and carries stress of 2 belts.
The idler would be 2nd suspect. If the pulleys have grease fittings, shoot some grease in them one at a time to see if the squeal subsides. If no difference, try to spray some WD40 under the idler to see if the squeal subsides temporarily (need I mention with the engine off). I have a 7 year old craftsman and last year I had to replace shaft bearings (discharge side) and the idler respectively.
Also worth mentioning is that if you have to replace the idler assembly, make sure you get an exact replacement. If you'll notice, the top lip on the idler pulley is curved upward to keep from chaffing the belt when the deck is all the way down and the belt is at its greatest angle.