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 Understanding Your Cat's Vibrating Tail Behavior A cat's tail is a powerful form of communication that reveals its emotions and intentions. When the tail is erect and twitching, the cat is usually excited or happy, while a straight up Anyone else's cat do this? She vibrates her tail when she is happy to see me : r blackcats Quivering, Wagging, Puffed Up: What is Your Cat's Tail Trying to Tell You?