Houdini does it occasionally when he's got food and my arm or hand moves little inside the cage and isn't sure quite what it is (he gets fed after Harley, so my hands usually smell like her when I give him food). He also tail-slaps sometimes when someone new goes into the cage and he wants to be left alone. It's way different than when he rattles his tail, which he only does if he's frightened. This is more of an annoyed thing.