When the camera is shaken while switched off, a ‘rattling’ sound may be heard inside the camera, depending on the intensity. It sounds and feels as if a part is loose.
The GR III and GR IV have a built-in image stabiliser (IBIS).
This technology is based on a movable sensor that is fixed in the shooting position by magnets when the camera is switched on.
The ‘rattling’ and the feeling of a loose component are due to the design and are no cause for concern. This technology has been used in camera technology for many years without any problems.
It should disappear when the camera is turned on. If this is not the case, send the camera to GR Service. For more information, see this link...