I am guessing that your tach is electronic. Since you removed the Alternator, I would check all grounds to be sure that one of them isn't loose or dirty. Also even though you had the Alternator checked out, it could still have a bad diode in it that would cause problems once you put any load on it. I have had a loose belt before on some cars and normally the screeching doesn't go away after a minute or so but will happen every time you get on it. How easy does the Alternator turn when it is cold and you turn the pulley with your hand?
Ron
