Do you have any performance issues related to the alternator? Dimming lights under load? You could take some silicone spray and shoot it into the alternator to see if the squeal goes away. It could also be a belt squeal that only presents when the engine is under load. Spray a shot of belt dressing and see if it goes away. If you don't have any take bar soap and hold it against the pully side of the belt while the truck is running.