Could be a couple causes for this:
* the tensioner may need replacing - not keeping enough tension on the "S" belt * worn (stretched) serp belt - replace ~ every 50 or 60k miles.
it could be a serpentine belt or accessory belt.
i need a diagram for serpentine belt for 1990 gmc 350
Change the belt
Check you serpentine drive belt and pulleys.
Replace serpentine belt on Chevy avalanche?HOW
you dont they are self adjusting with the tentioner pully...
Water from the road has splashed up into the engine serpentine belt causing belt slippage that makes a squealing noise. The sqealing is one of your engine assessories that use the serpentine belt to drive it.
serpentine belt loose
Worn serpentine belt and/or belt tensioner.
Either it's too loose or you need a new belt.
Replace the belt.
sounds like you need to replace the serpentine belt