Why is there a rattling sound when I shake my mobile phone?
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You might hear rattling when you shake your device or tap on one of the corners. It is an expected behavior caused by a certain component inside the phone.
Your phone's camera uses "optical image stabilization." Since the lens unit can't be fixed when the camera is not working, when the smartphone is shaken, the unit hits the cushion structure in the camera unit and makes a rattling sound. It is a mechanical specification and is not a malfunction. In addition, thorough product reliability tests have been conducted, and this phenomenon doesn't lead to failure.
You don't need to worry that something is loose inside the phone as long as the unit has not been abused, e.g., dropped, thrown, etc.
What is the optical image stabilization function?
This function keeps the lens unit, which consists of everything from the lens to the image sensor, in a floating state and moves the entire lens unit to correct the camera shake and reduce image blur.