Can you feel down at the base of its neck/breastbone area? If it’s a female, there will be a deflated sack. If it’s male, it will just be breastbone. Ultimately, the best way to sex an emu is by sound. Males can’t and don’t drum or boom. Only grunt.
I’m with Pyxis in this one. The description of the sounds it’s making, lean more towards female than male. The males grunt and growl, as they don’t have the big pouch that the females have for drumming.
Happen to have a video or recording of the behavior? Both males and females act a little...