I just message Samsung and got this response: Your device supports an Auto Focus function, the Camera module lens unit will be suspended over the image sensor. The auto focus adjusts by moving the lens unit up and down over the image sensor. This helps the Auto Focus speed and accuracy and also reduces battery usage. The camera module is fitted with a 'Stopper' to limit the lens movement within the camera module - when the lens and stopper touch (by shaking your device) a noise may occur. This is a normal operation of the camera. So it doesn't rattle when the camera is on. I am not gonna worry about it as everything works just fine.
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