Hi,
My girlfriends' car ('97 LeSabre) did the same thing the year after she bought it used in 2003. I investigated the problem and got many different answers on how to repair. GM Tech over at
www.aircondition.com told me it was a very common problem on the GM V-5 compressor(Harrison) and it just needs to be adjusted. The symptons are: Noise when compressor is not engaged. Only when the pully is turning and not the clutch. It scraps because of a lack of air gap clearence.The clutch plate rubs against the pulley. You need to go to a local auto parts store such as Murrays or Auto Zone and borrow a compressor clutch tool kit. You can then back out the clutch plate to .020 air gap from the pully and this will solve the problem.
Many shops will try and charge you for a new compressor, or at least a new clutch etc. I was quoted from $200-600 for these repairs and it ended up costing me "0". When the air is on or the compressor is engaged the noise won't be there because of the meshing of both parts.
You can do this job if you can work with common tools. Save yourself alot of money and stay away from the shops on this one. I've done alot of work with a/c but wasn't familar with this problem until GM Tech helped over at that website.If you go there you'll see some of his posts concerning this problem.
Hoped I helped.