I've had this for the last 2 or 3 years. It's a low pitched drone - a bit like a swarm of bees. At one time I thought my roof space must be infested!
It's nothing to do with power as it still happens if we get a power cut.
It's not industry or roads as I live in the middle of nowhere.
I'm not built over a gas main.
I've heard all of these possible explanations and none of them apply to me.