If i understand the purpose of the Idle air control valve correctly on this vehicle is that it controls the idle rpm of the engine by finely controlling the amount of air going into the intake manifold electronically. If so i would assume that while the gulping sound is there the engine rpm would fluctuate? Can you see or hear that happen?
The only thing I can think of, and this out of the box thinking on my part, is that you have a faulty Mass Air Flow sensor (MAF). the MAF is sending erroneous data to the ecu and because of this the IAC is fluctuating between more air and less air and thus creating a gulping sound. This is my wacko theory, and yes this gulping sound is quite strange to me.