Your compressor is not running very long - this can also indicate a state of low refrigerant. The sound could be a slipping clutch, but it could also be the compressor itself.
The compressor runs obviously with the A/C on, but it also runs with the defrost on, due to the fact that cold air is much more effective at defrosting a windshield than is hot air.
As for the gas tank, change your cap!
__________________
1988 9C1 - Modified LM1 @ 275HP/350TQ - TH700R4 - 3.08 8.5" Disc Rear - see it at
http://www.silicon212.org/9c1!
2005 Crown Vic P71 - former AZ DPS - 4.6 liters of pure creamy slothness!
1967 El Camino L79/M20 old school asphalt raper
Remember - a government that is strong enough to give you everything you need, is also strong enough to take everything you have.