Auditory phenomenon is pretty common in these scenarios. I used to have similar experiences when I was around 18 or 19. I heard family members walking into my room, but they were all at work, so it freaked me out. The vibrations would happen pretty regularly and once i got so freaked I rolled over and out of my body onto the floor and the whole thing evolved into what people refer to as an out of body experience (as stated on the first page). I usually associate the phenomenon with vibrations and a load roaring sound, as well as people's voices. When it occurs now, I've gotten pretty good at turning it into a lucid dream. It's no fun if you don't know how to manage it.