Loud rumbling in a clear sky. Many people in area heard it.Me an my grandson (he's 18yrs old) were working outside our house on this day and time when, from a sunny and clear sky, came a sudden rumbling or "quaking" that seemed to come from directly overhead and was very loud (like thunder) and could be heard echoing off the nearby mountains. It lasted between 5-10 seconds. I am ((deleted--sixties)) yrs old, retired from the military with ((deleted--20+) yrs service, and I have never heard anything like this from any kind of aircraft. The closest thing i could compare it to from my military experience would be the sound of a Vulcan Helicopter machine gun. It came of in an instant like the flip of a switch and ended just as abruptly. Several people we have talked to in this area heard this and so far no one has said they saw anything in the sky when it occurred.