If the belt and tensioner are both ok (and I doubt it) pull the belt and make sure each accessory is spinning freely, smoothly and is not too hard to get moving. Does it squeek as the AC compressor goes on too?
Wild case possibility, it could be a starter bushing or the squirrel inside the starter making a screeching noise. The Bendix could be wearing out, even if a slipping belt is much more common.