fluctuating hum in neutral and park

[clintryplug]

Just got a '97 Pathfinder (81K) and I was noticing when I got home that there is an intermitent fluctuaing hum (kind of like a Star Wars light saber) when the car is idling in park and in neutral with the A/C on, when I turn the A/C off (even with the fan still running) the hum goes away. Anyone have any ideas what it could be?

[clintryplug]

had my mechanic check the hum, said it was definitely the a/c compressor, not a good sign but not to worry about it until it quits blowing cold air. Gotta love a mechanic who tries to talk you out of repairs as opposed to ones who try to talk you in to them.