Well, now you know. I've heard it three times over the last thirty years. (we have a lot of mose in these parts). I'm not sure how they DO it as I've never seen them do it (just heard it while the moose would theoretically be maybe twenty - thirty yards away or closer?), but I'm told they just puff air out their lips with their snout down to the ground, kind of like you would hold your lips closed tightly, then all of a sudden let the air out to make a popping sound.