Check the tension on your fan belt. With strong fingers, you should be able to push / deflect the centre of the belt 0.5". To adjust, slacken off the three pivot bolts (two at the bottom, one at the top) so that the alternator will move.
Hold the alternator body so that the fan belt is tight, and re-tighten the top bolt. If the tension looks good, then re-tighten the lower bolts. If the fan belt is loose, it will slip and make that horrible shrieking noise.
My fan belt gets quite noisy at times and I have found that if I spray some 'belt dressing' on it quietens down dramatically. This is available from all good car shops.