Because the throttle body's valve has to open to suck in air so the engine can work, so when you floor it, the valve opens and you hear the air coming into the Intake
. When idling the valve is only creeked open so you hear a faint sucking noise at idle. Plus air has to be HITTING and passing through the Intake
to make the noise, at a stop there is no air pushing into the Intake
as it was in motion.