Dunno, maybe a leak around the IAC. The computer opens and closes that valve when the car is below a certain speed (30 mph, I think?), so it could be opening and the gasket whistling at idle.
Could also check the throttle body gasket while you're in there. They get brittle over time. This seems less likely since that should always be leaking, although it's possible.
Could also be a vacuum leak somewhere but you would probably notice that with rough idle and poor mileage. If you've had the problem for a long time, you don't notice.
Alternators will make a dull mid-range whistling sound but they don't come and go with idling.
Good luck