While you have the belt off, check your harmonic balancer to make sure the sheave is still vulcanized to the rubber damper.
I recently had a squealing/squeaking noise on another car that I attributed to the water pump. It turned out that the sheave on the crankshaft balancer had become unstuck from the rubber that is sandwiched between the pulley hub and the sheave.
It would squeal on startup and then catch after a little while.
