Any rubber dust/shavings? if so something misaligned. A washer behind my alternator shimmed mine back inline and fixed repeated belt squeal. That was on a Cummins Diesel I installed in a old Chevy 4X4 though. Is the close or far side of the belt shiny?
Spray of most sorts will quiet it of course, but a bad bearing might also cause alignment issue and squeal. And too tight belt tension cause issues in time. But that was usually cured with a spring style unit.
I know its tight in there, but a ruler or straight edge accross the pulleys can give you some idea of the surface plane of the pulley's.
Good luck!