As the tensioner gets older, it can wear on the bottom and the gap at the top starts to open up. This allows the pulleys to run on a slight angle pushing the belt across and causes the edge of the belt to chirp as it runs into the lip/edge on the next pulley. As it gets worse the edge of the belt will strip off too.
check your tensioner with a straightedge across the face of the pulley, and compare that to the other pulleys.
Explained pretty well HERE (better than I just did;)) and more detail.
Stevo

