I feel the two different types of vibrations that have been mentioned here; the one that feels like an internal "tuning fork" and the other that feels like an earthquake. I've felt the tuning fork vibration ever since I was a little kid. I was terrified of it back then and for several years, I supressed it until about 5 years ago, when I started "awakening" or becoming more aware of ...hmm, everything new that is 'new' to a typical sleeper physical-bound human. (I'm sure everyone knows what I mean
I usually get the vibration just as I'm awaking from sleep. I'll lay there still and feel like ripples or waves of electricity are pulsing through my body. I haven't felt it happening for more than a minute at a time, and never during activity.
I feel it when I'm falling asleep, during, and while awaking. It takes me several minutes before I can physically move because if I do it too fast, it feels like such a shockwave that I'm afraid for my heart to stop.
It wasn't until about a year ago I started feeling the "earthquakes". I thought I was going a little nutty when I had to look around if other people felt it and no-one seems to notice. I'm glad to find other people with this too!