Could be the belt. My brother has an 02 GMC that has the thread on fan clutch. Many years ago we didn't have the special tool to hold the water pump pulley so we were pushing on the belt with a block of wood to try to keep the pulley from slipping on the belt while we tried to break the fan clutch loose. We ended up damaging the belt, though it wasn't visually obvious. It took us awhile to figure that one out. look at profile of the belt all the way around and look for a 'wrinkle' in it.I ordered a new alternator for the Pig. I've got a chirp under the hood, and that's the only spinny thing that isn't new.