Is it from the wings dragging the ground?
I've heard it a hundred times when a turkey is strutin, and I know where the "drum" sound comes from.

No it's not from the wings hitting the ground.He does jab them into the ground simultaneously when he spits,but he actually emits that sound from air coming out of him.I have seen their snoods jump when they make the spit,so I know air is coming out of them.I also read an article on here that said biologists now agree that the spit and drum are sounds from deep within the birds chest..Hope I helped.

I agree. I have watched my pet turkeys here at home and know its coming from the air exiting. Seen it hundreds of times...