Both of the cats that I've known as young'ns did the growling thing with their toys. My college roommate's cat, as a kitten, would carry around and growl at his favorite mouse toy. And my own Rizzo was another wand-toy fiend. We had many a tug-and-growl match, and he broke several of his favorite wand toys. When I didn't pull back on the toy, he would prance off with it like he was carrying his kill back to his lair. He still does that part of it. Which makes me very happy, as it's terribly cute.