A few times I've heard a rapid tapping sound coming from Nibbler's direction. Like he's tapping on whatever he's standing on. (i.e. plastic cage bottom, plastic mat under the cage, etc...) It's really fast though, like he's itching and maybe tapping his other foot. I've yet to witness it with my eyes, so I'm not sure what he's actually doing. All I can think of is Thumper from Bamby where he's scratching and his other foot gets going. But it could be something else. Has anyone seen a rat do that? Or something else that would make a rapid tapping sound?