I had a cat when I was growing up that would do that to my mom and me and finally he found a blanket my mom made and he would suck on that. Like a baby sucking it's tumb kinda thing. He would knead it and lick it and carry on. The blanket had little braids in it so it seemed like a place to nurse for him. But if we washed it he would get mad and attack us for washing it. He liked the scent of it and being able to keep the other 20 cats away from it was his priority.