My husband found this on BBC...He knows I have had The Hum
on and off for about 5 yrs.

I hear it mostly on vacation. We are isolated, no electricity there.
I always thought it was something coming over the water.
An engine or perhaps some generator at the light house.

But I have heard it at home, sometimes too.

Here is the article. For a while The Hum was in western USA, in
New Mexico or Arizona. I've never seen a story about The Hum in UK before.

http://news.bbc.co.uk/2/hi/uk_news/8056284.stm

I have had two versions of The Hum... one note and two notes.
Since I have been using a nitrate drug to improve circulation and lower my blood pressure I have not heard it at home. The real test will be on vacation this year! I also hear it mostly at night. Rarely during the day when I am active.