Are you sure the tensioners are fine..... have you tested them?
I had a garage initially swap out the belts, but a few months later the squeal came back followed by some rattle can/clacking type noise when cold and in the end both a main serpentine belt and super charger tensioners were replaced. The first garage missed that the tensioners were gone, or they were just about to go.
I have a thread with some videos on here somewhere.
