maybe there's a problem and it's cycling the a/c compressor when it's not supposed to (i.e. not in a defrost setting)... try unplugging the connector to the compressor and see if it still does it. (but remember to plug it back in afterword)

Goin' Round Traffic Circles @ 50Km/h!!! \m/ -- 1998 E0 SVT #2119 of 6535 \m/ -- 2003 Sentra SE-R Spec V