This female is using her vocal sac to 'boom.' Although she's facing away, you can just see the 'swan-neck thang' going on. These are quiet booms. When they really crank it up, they go into a sort of almost-half-crouch/swan neck. From side on, you can clearly see the inflated sac in the chest.
So, whatever other noises any and all emooz make. If one of your emus is making this boom, she's a she.
Can't find the photos I'm looking for. This is Felicity. She is aimed at a female commanding territory to the south west. She has her ruff up. And in a second, she'll drop into the Serious Booming Position for an 'interterritorial boom' (rather than, say, happy emus in the early a.m. booms).