The cool, subtle vibration you described and witnessed may have been due to testicular circulation. Veins and arteries allow the blood flow to run through the entire scrotum. If your boyfriend's testicular movements were even more pronounced, then you can thank his cremaster muscle. It gently bungees the testicles to whatever part of the neighborhood they need to hang comfortably in. Another muscle, the dartos muscle, helps move the testicles up and down inside the scrotum.
Testicles are temperature sensitive sperm factories.
Sperm need to be kept at a certain temperature to be happy and healthy. Feeling stuffy, they can hang lower to get cool. Touch the upper inner portion of your boyfriend's thigh. His testicles will move in the direction away from where your hand is touching. This response is known as the cremasteric reflex, which is a safety maneuver that the testicles perform to stay out of harm's way.