Sounds like a roller bearing is failing. One of the oldest tricks in the book: BE VERY CAREFUL when doing this. Take the longest screwdriver you have; Place the metal end just behind the pulley, and on the bearing housing of each of the engine accessories and tensioner, then carefully put your ear to the handle (don't let it slip off or get into a belt!!!). The screwdriver will act as a stethoscope, amplifying the bearing noise. This was taught to me by my Grandfather who was a mechanic in the 30's thru 50's when diagnostic procedures were done in a more artisan fashion. It does work and although free and effective, it can be dangerous. If you want a safer method, buy yourself a mechanic's stethoscope like one of these:
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=mechanic%27s+stethoscope
All I can say is if you have a bearing howling or squealing you will be able to isolate it best by sound. This condition will not be a problem for long, because if it's a bad bearing, it will soon lock up and rub the belt in two. Other possibilities are: A/C clutch slipping, loose and slipping belt (usually from some accessory being too hard to turn), PS pump very low on fluid or failing. You just have to get under there and find the problem. Remember, the smaller the (driven) pulley, the higher RPM the accessory runs in relation to the drive pulley (usually the Balancer). Point being, things with smaller pulleys have many times more revolutions then things with larger pulleys. Bearings on smaller pulleys will fail first (usually). The sound is most likely metal on metal, and hardened steel at that. Best of luck to you!