On my old '97 GTP, I had a belt squeak that would not go away. There was no alignment issue I could see, my assumption is that it was the back side of the belt on one of the pullies. Car originally had a Gates on it, but it was old and in need of replacement. Went through two Daycos before finally giving up and just trying a (at the time) Gatorback. Squeaking never came back for as long as I owned the car afterwards.
At this point all I run on my cars are the Continental Elite belts.