What your snake is doing is very normal in many snakes, especially Kings and Milks. Vibrating/shaking tail is a sign of agitation/nervousness and is a defense behavior. I have witnessed this in babies in all of my time keeping snakes, and with some adults. Nothing is wrong with your snake, and it is quite normal. I have seen snakes of mine who never did this before, out of the blue, do this, and it was just simply them being agitated. Very normal!
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Genesis 1:1