What is source of mystery boom?

Can any of your readers throw any light on a noise I have been hearing for several weeks?

It is a kind of boom, not the sound a shotgun or bird scarer makes, but deeper. It usually sounds in groups of three, perhaps 20-30 seconds apart, then nothing for half an hour or more.

It sounds as though it is a good distance off, maybe several miles, and is virtually impossible to fix on the direction that it is coming from.

Anybody living nearer the source ought to have an idea of what it is. Any help would be appreciated.


Kerves Lane, Horsham