Sounds like belt tensioner arm bushings are worn. Does the edge of the serp belt ride over the tensioner pulley a little bit? I had a squeak and tried three different kinds of belts, including the gatorbelt. It was a worn tensioner arm bushing and bolt.

__________________
2009 E320 Bluetec 117,000
1995 E300D 306,000 Sold
1996 Ford Taurus LX 130,000 Sold
1985 300TD Still 225,000 Sold
2016 Ford Fusion 24,900