It looks like they are vibrating. When a cat's tail shoots straight up and quivers or rattles at the tip, it is the highest form of excitement they can show. It's a A Guide to What Your Cat is Communicating: A cat's tail is far more than just a balance toolit is a sophisticated emotional barometer that broadcasts their internal world. When your feline Why Do Cats' Tails Puff Up? Causes and What to Do Kinship Why Do Cats Vibrate Their Tails? Reading Your Cat's Body Language Kinship Veteris Cat Tail Shaking: Is it an Emergency? Veteris Why Do Cats Wag Their Tails? Cat Explore