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 Cat tail signs: What your cat's tail tells you Why Do Cats Vibrate Their Tail? This Is What They Might Be Signalling With It We Love Catz Why Do Cats Vibrate Their Tails? Reading Your Cat's Body Language Kinship Why does my cat shake its tail like this? : r cats