I have a 1998 GT and am experiancing the same thing after replacing my serpentine belt because the old one was frayed. The original belt did not make noise.
I replaced both pulleys and the water pump with no change in the noise. I spent many hours reviewing postings on this sight and on others for the same problem. No one would definitively say what caused or fixed the noise.
I believe it is caused by misalignment of the pulleys or the amount of bending (backwards the belt has to do over the water pump). The noise was bad on my car yesterday, I went home and sprayed silicone spray on the flat side of the belt only. The noise immediatly stopped and would not return even when revved.
My path forward is to
1. Continue to drive until the belt "breaks in" or
2. Replace the belt with the Goodyear Gaterback. The reason I think this would work is because the Gates belt I bought at NAPA has continuous ribs on the inside of the belt. The Goodyear belt breaks up the ribs so it is easier to bend the belt backwards.
Has anyone fixed this problem by using the Goodyear belt?