I think I had the same issue as you. After researching on the net I found a few people that said the OEM belt would stretch after about 500 miles and start to squeal. The info I was getting was mostly concerning 1999 to 2002 F53 chassis. Since mine is a 1997 F53, I decided to check it by noting the position of the tensioner, then I removed the belt and noted the position without the belt installed. What I found was the 2,000 mile OEM belt was near 90% at the end of the tensioner spring travel. This will cause a weak spring tension on the belt and will allow slippage as the serpentine belt stretches.
I then noted my OEM gates part number and went to gates.com. Using the part number info on their site, I found the next size smaller belt. It was .4 inch smaller in diameter. After installing the new belt (it was a tight fit) I found the tensioner to be riding within 50% of it's travel. During my road test I could hear the difference right away.
Hope this helps
'98 Gulf Stream Sunsport 325, 7.5L Banks Power Pack, Koni FSD's, Air Bags, ReadyBrute Elite,
2000 Honda Accord