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 Why Do Cats Vibrate Their Tails? Reading Your Cat's Body Language Kinship What is she trying to tell me? Vibrating tail??? : r CATHELP What Does It Mean When A Cat's Tail Vibrates? Why Does My Cat's Tail Twitch? 6 Vet Reviewed Reasons Catster How to Read Your Cat's Tail Language and Meaning PetMD