You might be interested in this article:
Spontaneous Middle Ear Muscle Activity in Man: A Rapid Eye Movement Sleep Phenomenon

"Middle ear muscle activity typically erupts before or at the onset of rapid eye movement (REM) sleep and persists throughout the REM period in a discontinuous pattern resembling that exhibited by rapid eye movements."

I think the vibrations are due to the inner ear spazzing out during the onset of REM sleep. This could also explain some of the disorientation, spinning, falling, bending, and other balance related strangeness.