Grinding noise under the hood

1997 Buick LeSabre Limited, 83,335 miles, 3800 engine. After the engine has started, I hear a grinding noise in the engine compartment. The noise stops "after" the defroster has been turned on. Is this a compressor or a blower motor issue? Also, I've been quoted compressor prices ranging from $200 to $400, which does not include labor. Would I fare better by purchasing the compressor from an auto parts store, and having a shop mechanic install it? Labor rates range from $47 to $60 pr. Hr. HELP! Thanks!