Here's a long overdue follow up.
I finally cracked open my Lumia 900 after breaking the screen and needing to replace it. I had nothing to lose since AT&T wouldn't cover the phone's screen and insurance deductible was the same price as the part...anyhow...
The rattle is 100%, unequivocally, without a doubt and with 6sigma certainty coming from the vibrator mechanism soldered to the main board. This mechanism sits directly to the side of the camera mechanism.
With the phone 100% disassembled, I was able to manipulate the parts in my fingers and test each and every parts the culprit. It is the vibrator motor and it cannot be fixed without extreme measures, like desoldering it from the board for inspection.
The cause is as simple as a crappy part Nokia OEMed from someone else.
BTW...totally different mechanism from the 800. After having been inside both, its pretty clear to me the 800 is a MUCH higher overall quality device than the 900. If the 800 had LTE and FF cam, it would kill the 900.
Anyway...bottom line: if your phone rattles, it is not user serviceable. Take it back.
If anyone wants pics, lemme know and I'll post them.
Nokia Lumia 920