Hey. My first post! To OP, my launch day 6 was rattling too. Took it in and they swapped it out for a new one which doesn't do it at all. My original phone seemed to work fine despite the rattle but I don't think it's normal. Might want to swap yours out too just in case.
Sadly, give it a day or two for psychological factors to do their thing, and your new phone will "rattle" too.
Unless, we're talking about something different, (which is possible, I'll admit) the vibration from the camera when setting the phone down on a solid surface or tapping the phone near the lens, (even if you hear it from the sleep/wake button) is normal.
So normal, my BOL's Samsung gs5 does it, and after a friend said his Moto Razr Max didn't do it, I showed him it does.