Does the noise reduce after a couple or so minutes ? 
Has your Air Con been serviced ? 
 
I've had a similar problem with my Focus II Air Con. It sounded like a bearing noise at first and then sometimes it whistled for a short time but then most of the noise reduced after a couple or so minutes. Since I had the Air Con serviced during the cars last service the noises have gone away and the Air Con runs quietly now.
                            
                                Edited by MikeTorque on 23/06/2008 at 11:33
                             
                            
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