It usually only lasts about 2 to 4 seconds, but if you know what im talking about, it feels like a faint blast in the distance that vibrates your house, sometimes you might even hear a humming sound coming from portraits on the walls, or a bottle of water vibrating on the beds headboard when it happens. I live in a state with no earthquakes, and a small town with no trains, planes, semis, heavy traffic, or industrial plants, etc.. that could do that to my house. My town gets spooky and quiet at night, so just want to rule out any plausable factors. But yeah my house will shake at random but only at night.