Hello, I hope you folks can help me.

Recently, we have been hearing a loud, low frequency "hum" throughout our house. I have turned the main power off to determine whether it's electrical. It's not. The noise does sound like a "fog horn" going off several blasts at a time and the noise level is constant in every level/room.

I think this is a water pipe humming problem like other people have described. However, I have turned the water main off and the problem does not go away! A quick check of indoor faucets and toilets didn't really turn up the culprit.

I recently got a lawn sprinkler installed. I can't really check its pipes. Could this be the source of the problem too?

Before I call a plumber, is there anything else I should try? Attic/roof inspection? Gas line? Structure?

Any suggestion would be appreciated. This noise is very annoying.