The Hum. If you played a very low sound gently through speakers, it sounds like that. The frequency is about 20hz, I'd guess. Occasionally strong enough to vibrate walls and window panes. I can only hear it at night, and only in certain locations. I used to think they were earth tremors, because there's quite a resemblance. But they weren't reported as tremors online, and in any cases, they came in bursts of about 5-15 seconds, but lasting hours. Other people can't always hear them, which can be a bit disconcerting! Apparently only 2% of people can hear the Hum.
There's a Hum map online. People first started hearing it in Bristol UK. Lots of explanations out there.
