Yup it's this. turn on the camera and shake your phone, it should stop
It may not stop just by turning the camera on. It has to be actively using the lens (i.e. actively focusing) for this to stop. You can do some tests by doing some repetitive tapping and hear it then tap the screen with your other hand and allow it to focus as you're tapping to duplicate. As it's focusing, you won't hear the rattle nearly as much if at all. That's how you know it's the lens.
If you're like me however, your sleep/wake may be adding to the sound and so you may not be able to silence it 100% during testing unless you were born with an extra finger that you can lightly touch the sleep/wake button with as to silence it.
At the end of the day, if it bugs you, try sending it back. You'll be in the same spot as me. It kinda bugs me since this thing is an $850 object and anytime I plunk down that kinda money, I want it to be 99.99% perfect.
At the same time, it IS a mass-produced device that is prone to other actual defects that may bug you (and me) more such as the black paint (anodization) chipping off or the screen being yellow -- which the iPhone 5 screens are yellower, light bleed, bad battery, etc...
I'm about 55/45 with keep/send back. I don't have OCD but I do have a heightened level of anxiety over normal people lol.