Quote Originally Posted by Robot_Butler View Post
The physiological explanation is spontaneous middle ear muscle activity. Your middle ear (tympani muscles) spazzes out when you start to enter REM, similarly to how your eyes go crazy (Rapid Ear Movement Sleep?). This is what causes the buzzing sound and vibrations in your head. Your ear drums are literally vibrating themselves.
Citations? I'd like to read more about this, as I've been trying to figure out the physiological mechanism behind the vibrational stage for awhile.