I have NO IDEA what that internal vibration is but I'm so glad to see some others have had this experience. For awhile I thought there was a short in the electrical outlet behind the head of my bed, which is butted right up against it. I jumped out of bed and pulled it away from the wall - to no avail. Those little electrical-like impulses still show up at night, from time to time. Not particularly "Good Vibrations" I'm afraid.
I do also have fibromyalgia.