There is one other belt there no one has mentioned and it has even more drag on it from accessories. I'm sure it's even more susceptible to slippage than the 'charger belt.

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I also have a very loud squeal between 2500 and 4000 RPM, both in neutral and when in drive. The local SVT dealer is looking into this now. they plan on replacing the belt and tensioners. I tried to buy the belt myself (part number JK8667 - 8 Ribs, 66.7 inches long), but they told me Ford was on backorder for months. i emailed the folks at Goodyear, Gates, etc. and only received an email back from Gates, who has a belt, part number K080660, which is 66.5 inches long (close enough, with the tensioner) which i am going to try while Ford decides what to do with th squeal. the squeal is so loud people actually turn around and look at truck. i hope the Gates belt is a better product than the original Ford part.