rumbling

Every now and again (maybe once a month?) I hear a loud strong rumble that shakes the house a little. I even felt it while sitting out in my backyard once. Felt like a little earthquake! Not sure what it is or how to begin diagnosing it. Any ideas? We are on a well. Thanks!

Hi folks, I would really love any help if you all can identify what's happening in my boiler. This video shows it all, with sound to help debug: https://www.youtube.com/watch?edit=vd&v=fY7rAYZu05g The boiler is on natural gas, I recently cleaned out the chamber, replaced the rear refractory...