With that many miles on your vehicle you probably have a replacement ac compressor belt tensioner, like mine (2003 Tahoe with 344,000 miles). Most of the replacement tensioners seem to be made in Canada, probably by the same manufacturer. I went through 3 of those, one even from a dealer, and they all whistled at certain rpms in cold weather in Colorado (below 20 degrees or less maybe?). I recently looked into replacing just the bearing in one of them and found that bearings have low and high temperature limits. The ones with the greatest range are more expensive. I am guessing that the manufacturer uses cheaper bearings not good for low temperatures. Before I could get around to replacing the tensioner bearing I found another belt tensioner at Oreilly with a different design and manufactured in India (Murray brand I think) and so far it does not make the whistling noise. Don't know if this is your problem but somebody might be able to use this info, if not you.
