2) It's amazing how perspective changes with more information. It is very odd how much that sounds just like a mower having difficulty starting, and is exactly the conclusion I would have jumped to it being. Glad you were able to get a better answer.
Yeah, I thought it was a generator or mower at first. It's just quiet enough that I mistook it for a much louder sound that had grown faint due to my distance from the source.
I went deeper down the rabbit hole, and it is their mating call / territorial signal. The drumming sound is actually created from a vacuum effect caused by the particular movement of their wings. https://www.youtube.com/watch?v=vERkh23mp64
