I returned from travelling around the world 6 months ago and moved into a new house. It was summer and very hot so I had the windows open. We're now living in a more suburban area compared to where we previously lived in the sticks. For a few nights I kept hearing what I thought was distant traffic on the roads. So I closed my windows and got use to it and put it down to a combination of jet lag and a new house.
A couple of weeks ago, and now that we're in the winter with all my windows firmly shut, I couldn't sleep for 2 or 3 nights in a row. I could hear a faint hum, like traffic, or as though someone had left their engine running, or some sort of generator but I know that nothing was running. We now live in a village so it's pretty obvious when there's work being done. I am very sensitive to noise at night which is why I turn everything off. Any lights or fan noise from a PC will keep me up, so this 'hum' was horrible. I only fell asleep through sheer exhaustion.
The wiki link posted above describes the noise perfectly:
The essential element that defines the Hum is what is perceived as a persistent low-frequency sound, often described as being comparable to that of a distant diesel engine idling, or to some similar low-pitched sound for which obvious sources (e.g., household appliances, traffic noise, etc.) have been ruled out. There are a number of audio reproductions of the Hum available on the web.
It's probably just my hearing going. Poor me.