They all have this. It is the power button cap. It is not specific to your phone.
I am holding the power button, and ALL buttons, as I gingerly shake the phone forward and backward (if you are looking at the screen). The button cannot rattle if I'm holding it in place. I've even held the button down while I shook it, which, to me, rules out whatever part beneath the cap might be shaking.
I will post a reply once I inspect another unit at the store, after work.
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