Yeah I can confirm that there is some other parts also rattling than buttons. This rattle appears when shaking phone to certain direction or tapping top of the phone (if you know what I mean). It seems to be camera lens or atleast the rattling seems to come from somewhere near camera.
Edit: the noise is indeed the lens. Just tried to tap it when using camera and there is no rattle cause I guess camera somehow "locks" the lens. Anyways, these devices are not made for unnecessary shaking and searching for rattling sounds. Just interesting to know the source of that little "rattle".