personally i think the 'swoosh' is more felt that heard. the sound it makes, if any, would be so faint that you have to be in a completely quiet room to notice it. you should feel a sudden gush of wind from the sides of the humi as the lid drops down. nevermind if you dont hear the sound so much, but if you feel the air coming out, then thats what you're looking for.

hope this helps.