I had read that with a collapsed trachea the mouth would be open and with reverse sneezing it would not be. His mouth is always closed and it does SOUND like an inhaled sneeze. Man, I really hope it isn't a collapsed trachea!!
Usually when he makes the sound it's after he runs around with the "zoomies" and is playing really hard. Now that I think about it, it may happen 2 times a week or so - not really sure.