Pooky has done that since he was a kitten. Because of fear that he would start spraying I had him & Bear neutered at 5 1/2 months old instead of waiting to 6-7 months that was recommended. Pooky still does that and at no time has anything come out of him. I've since discovered that when he's really, REALLY happy is when he does that. It's like he's so happy he just can't contain the shake in his tail. lol