Worn, loose, or slipping serpentine belt. Also, a worn pulley bearing can allow that pulley to be tipped. That misalignment causes the belt to slide sideways across that pulley as it goes around it. That will cause a horrendous squeal.
Try dribbling a little water on the smooth back side of the belt while it's making the noise. If the noise changes, it's belt-related.
Tuesday, October 2nd, 2012 AT 5:56 PM