[pb060]

It could also be the tensor tympani muscle tightening in response to a visual event that would normally be accompanied by an extremely loud noise.

[Flenser]

That's also my theory. I can do it at-will by tensing my jaw muscles and I hear a "thudding" vibration sound. (Although I wouldn't have known what the name of the muscle is until your post.)

[pb060]

I can't do it at will but few times I was waken up by the "sound" of it vibrating, quite a scary experience!