Agreed!! This bull was giving you what I call the Popping/Nervous Grunt, it's a very common sound used by all elk, it was not a warning bark!! He wanted a visual or identification of you, he wanted you to come over to him now!!!! A warning bark is done in 2's & 3's & get further away quickly as they scatter, other elk can also chime in issuing the warning to the herd. There are ways to overcome this action, a bugle is not needed in many instances but I too prefer one! A few distressed cow sounds & some voice growls, moans & groans as you retreat from the bull can go along ways to ones success.
ElkNut1