It's the camera lens. It's normal. They all do it, some just to a larger degree than others. Don't believe me? Go to your local carrier or apple store and check the display models. They will all do it to some degree. You'll find some to be very faint and some to be really loud. It's not gonna hurt your phone and will not hurt the camera or lead to premature wear or anything. If your concerned, go trade it for one with a yellow screen or chips or loose buttons or bad battery or a slew of other things that your bound to find if you look hard enough. This is how a lot of non-smartphone camera lenses work too.
I returned my first 4s out of paranoia of the same issue. The second one rattled but much less. After a year of normal, if not gentle use, it's increased in rattle a bit and still takes amazing pictures.
My 5 rattles cause of a loose sleep/wake and the camera lenses. It takes awesome pics too. I'm not sending it back simply because I know the next one is prone to do the same and possibly have other cosmetic or functional issues. Plus ill be getting a new phone in a year anyway.
Anyone who says they can't duplicate the marble-like rattle by tapping the back just below the camera is either lying, has bad hearing, or honestly may have a stuck lens and their camera may have trouble auto focusing.
Most people just honestly don't notice it.