OK I went to a quiet room and checked mine. Holding the USB cover, and without the bat cover, I hear a really really subtle rattling sound when i shake violently the phone in front to back direction (not side to side or buttom to top).
Then I did the following in addition to holding the usb cover.
My position: phone in the left hand hugging it with the screen facing the palm, the point finger on the USB cover, the THUMB is on the battery AND ON THE LITTLE pin-type contact that usually contacts the battery cover, on the left side of the back next to the bottom screw and the micro SD slot. Shaking - no rattling sound. The sound is coming from that pin, it is spring loaded and floating freely within its space. When you shake the phone it is just hitting its surroundings.
Non issue for me, just a design. Try it and let me know if this is also the source of YOUR rattling sound.