Those vibrations are very common during WILD. Like you've already figured out, it is the stereotypical kundalini awakening that many people seek through meditation. The early days of research into out of body experiences were also hung up on these vibrations. If you read any Monroe, Bruce, ect, you will see they do all sorts of ridiculous things to encourage this "energy." Think of it however you want. It is a very cool feeling.

The physiological explanation is spontaneous middle ear muscle activity. Your middle ear (tympani muscles) spazzes out when you start to enter REM, similarly to how your eyes go crazy (Rapid Ear Movement Sleep?). This is what causes the buzzing sound and vibrations in your head. Your ear drums are literally vibrating themselves.