Are you sure that's not the exhaust vibrating against something? If the bolts holding the various segments of the downpipe bracket have vibrated out and if you motor mounts are starting to sag, all sorts of odd, buzzy noises can happen.

You seem to be hearing it right where an exhaust-based buzzing is most likely.

The only time I've ever been able to hear the IAC on my Rex/Regina car when the engine is running was the time I had it plugged into the harness, but was manually controlling airflow with a gas valve. Without the sound-dampening of the rubber hoses, the IAC's whirring sounded like a bunch of pissed-off bees.

John