Which is the best definition for droning as it is used in the following sentence?

I was trying to rewrap my hands so I could go back to work when I heard a low droning sound sound. I stood up and looked around.I could still hear the noise but couldn't locate it. I looked high up.high in the top of the big Syncamore a breeze had started the limbs to swaying.a shudder ran through the huge tree trunk.