Originally Posted by The Fed
In your case it's the belt. In other cases it's the bearing in the tensioner. I've even seen squeaking from the PAINT on the outside of the belt make a squeak as it goes over the tensioner. In that case, since you usually replace the belt along with the tensioner many people think it was the tensioner when it was actually the belt. So with an ACCESSORY belt, if you don't see the tensioner jumping around (from a weak spring) try just replacing the belt first. You would never do just the belt on a camshaft timing belt job, because it's so much labor to get at it you always replace the tensioner. AND the water pump, if it's run off the timing belt.
I found this out the hard way with a Nissan PU -
I had the water pump go out around 50K - Nissan recommended I also replace the timing belt - they only up charged for the belt - I agreed.
At 75K the tension pulley went out.