From the BBC:
Some people in Bristol say they are plagued by a mysterious low-level hum that no-one can trace. […]
But it’s not the first time the hum has kept Bristol awake. In the 1970s hundreds of the city’s residents complained to the council that a strange noise was audible at night. Most of the experts drafted in put it down to factory noise, electricity pylons or tinnitus – while some of the more imaginative suggestions included the sound from flying saucers hovering over the city or secret military activity. Eventually, it stopped as abruptly as it began, but not before it had spawned reports of equally unidentified hums in other towns across Britain.
Then last year French scientists announced that they had solved the conundrum. It was, they said, the effect of continuous waves causing the ocean floor to vibrate. “We have made a big step in explaining this mysterious signal and where it is coming from and what is the mechanism,” said Fabrice Ardhuin of the Centre National de la Recherche Scientifique. But his answer doesn’t immediately explain why the sound was only around for a number of years. Or why it might have returned.
Something in the air: The Hum seeps into music too. Bass culture runs deep in Bristol …