This happened 2 times.
1st time: Early in the morning I was laying on my bed watching TV. My friend was on the ground reading foot ball magazines. Then, all the sudden my bed shook and made a vibrating sound! I jumped off my bed and my friend was just starring at the bed. It stopped after like 5 seconds. That was a year ago.
2nd time: I don't really know what happened, my little sis and cousin (who didn't know about the first time) described the EXACT same thing.