You need to correctly isolate the cause of the squeal which can be easier said than done. Having replaced the alternator and belts, I would definitely say that the cause is elsewhere.
You need to arm yourself with WD40 or similar and when the squeal occurs at startup, spray the suspect areas until the squeal stops.
I had a similar problem with my VW Golf diesel and spent months fiddling with the wrong belt.
Squealing = loose belt or seized pulley/roller. If the squealing occurs on startup and then goes, my bet is a loose belt. They are usually very loose to cause this problem, so just check all the belts in the car.
I had a similar issue when I fitted a new belt. Although it was the correct belt it did not seem to sit as deep in the “V” of the pulleys as much as though the section was too large. Cant rmember what I did now but think I may have tried another belt from a different supplier.
I have come across similar on other cars and the problem was the bottom pulley. On these “other” cars, the bottom pulley was a bonded pulley, that is too say that it had a metal boss on the crank shaft, then rubber was bonded to the boss and finally the pulley part was bonded to the rubber.
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What had happened was that bonding agent had failed and the pulley was able to spin on the rubber when the belt was put under load. In many case’s the pulley will fall apart but for a while the belt will keep the pulley in place and lined up!
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I am not sure if your bottom pulley is of this construction but worth a check.
Another belt squealed and snapped today. Was worse on damp days starting Had a new water pump fitted last time (1 year ago ) Could it be the alternator?
16 year old car = MK2.5 = yes, sorry to say. Remove belt tension and see if there is any issue with the alternator pulley rotating freely. Typically these cars seem to attract water under the bonnet and onto the alternator far more than the MK1. Corrosion occurs, accelerated by a galvanic reaction(aluminium to steel), the cases swell and split taking the armature and bearing off centre, finally seizing rotation to the point of belt squealing/snapping.
