clean it, clean it, clean it. Probably some gritty stuff in there on the belt or the pulley. The longer the dirt is in there the more it will sand away at your parts like sandpaper. If cleaning does not help entirely, spray it with belt dressing after it is clean and that will make it sticky again. If it is stretched like mentioned above, just replace it. But keep the new one clean
After mudding you always want to hit a power wash and blow out these key areas to save some headache;
1. belts (all kinds of noises can come from them with dirt in there...)
2. Nerf bars (if you have family and friends who slide the backs of their legs on them to get out)
3. The insides of each wheel and brake components (run w/mud on your brakes and you'll burn up your pads a lot faster) I've also seen a ranger that had a small stone get trapped between the brake shield and the rotor. Damn that thing would grind loud for being a little tiny rock! It will also make your wheels bounce like hell on the hi-way if you have extra wheel weights made out of rock/dirt clumps.
4. Front/back of the radiator. This is where all the heat dissapates, keep it clean and it stays cooler.
5. Drive shafts. Spray 'em both to prevent a stupid jumping wobble from a dried clump of mud left over...
Aside from that, you always want to check under your air filter after a day of crazy mudding to ensure has not sucked anything into the air chamber.