Does it sound like a real high pitched "nails on the chalk board" squeal?
The 1992 Bonneville did that. the compressor actually ended up shooting peices of metal out. that was fun.
If you turn the a/c on...yes, it will be fine. However, if is 30 degrees outside and you dont want the a/c on anymore...the noise will come back.not fun
Im pretty sure you'll be buying an A/C unit. i'd also suggest using brake cleaner on the serpentine belt, just to be sure its clean and no dirt is on it. Thats just my 2 cents.
maybe someone with more mechanical experience could clear this one up further for you...but that should put you in the right direction.