I don't use the vibration technique. I've found them to be a distraction, but I am familiar with the sensation.
My thinking is that the vibrations are coming from blood pulsing through relaxed blood vessels. All of our blood vessels are surrounded by smooth muscle to help control blood pressure and body temperature. When we go through the process of relaxing our bodies for sleep we relax the smooth muscles around the blood vessels making them suddenly really lax and malleable. The pulsing blood causes a ripple effect kind of like punching down on a water bed creating the vibration. The variance in the vibration's location and frequency is do to the location of the blood vessels that are relaxing and their diameter/length like the strings of a guitar or piano.
My thinking is that the vibrations are coming from blood pulsing through relaxed blood vessels. All of our blood vessels are surrounded by smooth muscle to help control blood pressure and body temperature. When we go through the process of relaxing our bodies for sleep we relax the smooth muscles around the blood vessels making them suddenly really lax and malleable. The pulsing blood causes a ripple effect kind of like punching down on a water bed creating the vibration. The variance in the vibration's location and frequency is do to the location of the blood vessels that are relaxing and their diameter/length like the strings of a guitar or piano.