I forgot if the supercharged 3800 engines have one or two belts.. If there are two, isolate which one is sqeaking..
Most often, if you spray the belt with lubricant and the noise disappears, you need to replace the belt.. There are belt conditioners they sell to spray on, but they suck and don't work. I know the belt only has 12000 miles on it, but sometimes they just wear. Here are my suggestions..
1) replace the belt with an AC Delco brand.. this is what GM uses and what I only use.. never had a problem.
2) when u take the old belt off, make sure the pulleys are free of dirt and fluids..
you can try this first before buying a new belt actually.
3) make sure the idler pulley is not worn and make sure the mechanic didn't leave any bolts loose.
this is how I would start off....
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