I've seen it mentioned elsewhere by people who found it's caused by the plastic casing rattling inside the metal 'rotation ring', but mine is not caused by that. It's the little black bits you squeeze to fit or remove the camera from the mount. They rattle up and down.
I've contacted customer support and have been told to expect an anti-rattle device of some sort in the post but by the sounds of it, it's designed to cure the rattling caused by the metal rotation ring being too big, and is nothing to do with the black squeezy mounting clips.
Any advice will be gratefully received, before the whole thing goes back to the shop.