Whining seems to be a warning to you that the compressor may be ready to seize. I'll be posting soon about my own aircon experience which has involved three compressors - the original which seized solid and threw the belt thus requiring a flatbed truck trip home at vast expense, the replacement compressor which unforunately was installed by a garage who failed to flush the system and so suffered the "black death" (also at vast expense) and now the third one which is running well. The second one started whining after a while as the black death worked its grim work. The new compressor does not whine at all.
My car has now done 40,000 miles and the seizing of the aircon compressor seems to be a problem that can manifest itself around that mileage (depends how much you use it, or perhaps not use it, ie leave the ATC in Econ mode). My advice would be to replace any whining compressor with a new one from the US as soon as practicable before yours seizes and the cost of replacement escalates. Have a look at techchoiceparts.com - our FS10 compressors (new, not rebuilt or "remanufactured" as the Yanks call them) are half price just now ($US130)!! Search the site for 4.6 li Ford Mustang GT Mach 1 2004.