1993 Mazda 626 Replace Alternator Belt Once Driving Everything Sounded Fine Now

Can't hear it from where I'm sitting. Let me run outside and listen. You might have better luck finding the source yourself since you're right there. Look at each pulley to verify the belt is not walking off to one side of it. If it is that pulley or the one right before it is tipped or turned a little. That will make the belt slide across it as it goes around it, and that will make a horrendous squeal. If the noise doesn't occur for you with the hood open you can usually cause a change by dribbling a little water on the smooth backside of the belt while the engine is running.