I just got back from three days of archery elk hunting with some friends that have archery bull tags for unit 6a north here in AZ. On Saturday my friend and I spent all day in the cat tail reeds in Mormon lake. There are HUNDREDS of elk that stay on the mostly dry lake bed. We were surround by elk for most of the day. The bulls were bugling and the cows were mewing. There was one other sound I have never heard from elk, kind of a hollow popping sound and there was lots of it. I don't know who was making that sound, the bulls or the cows. Has anyone heard this sound and can you explain? Thanks.