Parts Avenue

I've been having some problems with my Sunfire recently. Everything seems to be pointing to the alternator being bad but my mechanic (not dealer) said that it's fine. $200 later it still makes a terrible high pitched “sewing machine-ish” type sound. My dad (not a mechanic but knows his stuff) says he thinks the alternator is the problem. I guess I'm just wondering if other Pontiac buyers have had the same problem or if this sounds like a bona fide alternator problem. Any input? It would be greatly appreciated. Thx