ALL of my rat and kingsnakes do this. Very common. Pines and gophers do it as well. my Beauties are funny about it. They will vibrate they're ENTIRE long prehensile tail, instead of just the tip like my corns and kings.
:I opened the lid to my black rat's feeding container today (he's about 6 months old), and I guess it spooked him because he started shaking his tail rapidly. Of course on the newspaper lining, it sounded just like a rattlesnake. Is this common?