I had a very similar noise based on your description. It turned out to be the AC compressor starting to fail. It completely gave out right at the tail end of the really hot weather we had this past summer. With the brakes, suspension, tires, transfer case, and exhaust that all went bad since the last inspection, I didn't even bother fixing the AC, or anything else. Repair cost was more than what I owed, so I just traded it in.
2005 Kia Sorento LX 4x4 44,000mi
2003 Chrysler T&C Limited 3.8l AWD 145,000mi Traded in 9/14/11
2000 Dodge Stratus ES v6 179,000 sold 7/2011