I need a little help. I drive a 2003 f550 with the 6.0 and manual tranny. There is a squealing noise that sounds just like a slipping belt that is happening a fair bit now. Strangely, it only happens when the hvac is on defrost or a/c. You can actually turn the **** and turn the squeeling on and off. If it was just the belt, it wouldn't do that I don't think. Any ideas? Thanks.

 

I would check the belt, belt tensioner and the pulleys. You can remove the belt and spin each one and feel for excess play or rough movement

 

When defrost is selected, does that engage the a/c compressor clutch? Trying to understand why switching from off to defrost makes it squeal.

 

Yes, A/C runs in defrost mode. To help dry the air.