From my experience:
I' ve been close enough to them while they were still on on the roost and I' ve heard them drumming.
Mostly:
It seems to be when they are in the close proximity to hens or if they think a hen is nearby. Someday you will most likely call in a tom (no hens around) and you WILL hear him spit and drum! It is one of my favorite things about spring gobbler hunting. It usually precedes a very loud BANG!