Here's one for ya. After losing another Buick engine, I just converted my Buick GSX clone to a chevy engine(I know) for expense reasons. It is a rebuilt 350 warmed up a little. After it has been warmed up for 5 to 10 minutes, I get an annoying squeeling noise from the engine bay. It only has two belts and they are both new. One is the V belt frrom the balancer to water pump to power steering. The other is a serpentine from balancer to water pump to alternator. I replaced the clutch for the fan, and now I did the alternator. Everything is new. It still squeels. It doesnt do it at idle in drive but it starts around 750 RPMs in Park on up through the gears. Someone told me that the serpentine pulleys themselves can get worn and make a noise. And when I bought the new alternator, I had to put the serpentine pulley on it, as that style of alternator has an external regulator and they come with V belt pulleys. Any insight would be great, as Im going insane chasing this thing down.

 

my advice is to take the belt off completely and run the engine to make sure it is or is not the belt and possibly a main bearing

 

I've had the bearings in the serpentine belt idler pulley go bad and make a squealing noise.

 

Did it squeal before the pulley was R&R'd?? If it didn't squeal before the alternator/ pulley change, the alignment of the pulley may be off slightly. Look for wear on one edge of the belt inner/ outer. If one of the edges is showing any wear the pulley may have been pushed on too far or not far enough.
Good Luck,