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 Cat Tail Language: 10 Secret Messages Your Cat Is Sending You With Their Tail Chewy What does it mean when a cat's tail vibrates? What Your Cat's Tail Can Tell You Why Do Cats' Tails Puff Up? Causes and What to Do Kinship The Definitive Guide to Cat Behavior and Body Language Tuft & Paw