I have a odd issue at times, maybe I'm just weird but I can smell the difference between with the compressor and when it is not running by the way the air smells (best way i can describe it is it doesn't smell crisp) I use the automatic a/c option and leave it at 70º. I notice specifically after driving for about 15-20 minutes the smell change, about a minute or so later the crisp smell comes back. I tried a couple of times to turn it down to the "Lo" setting and no difference. Keep in mind I live in Texas and we frequently have 100º+ days in the summer. When I first noticed it I took it to the dealer and they said that it was a little low on freon so they topped it off and checked for leaks. No leaks found. That was almost two years ago so I do not think its compressor or refrigerant issue. Possibly programming......