Humming Hornets

This hornets' nest is perched high above the road we take to the lighthouse when it is too wet to take the dirt road. It is, as you can see, on the power line. Maybe they are trying to become fireflies?

In the zoomed in shot below, you can seen the little hole that serves as the entrance and exit. There was no activity when we took the pictures.

The question begs:

Why to hornets hum?

Maybe it is the electrical current.
Or  have they just forgotten the lyrics?