Just wanted to add... I have the same rattle. And it's pretty audible to me. I actually did some research online about this because I wanted to know if it was normal before I headed to the store for a replacement. As people have already said, it's definitely the camera. It seems the autofocus in cameras can make a rattling noise. And searching online for any information on an autofocus rattle will return results with this same issue on a lot of different phones. The iPhone 4S seems to have a big problem with this, as PhillyPhill39 mentioned.
So, it seems like a pretty common problem for cell phones. My mom's Droid 2 does it. I even checked my ancient Samsung Glyde and my grandmother's Samsung flip-phone. Both rattled.
I did talk to Verizon tech support a few times though. Two people said the noise wasn't normal. One of those people told me to get a replacement. The other told me it was the SIM card slot rattling (because she called my local store and that's my store's argument). The other tech told me if it's not having a negative impact on the performance of the device, there's nothing they can do. My local corporate store absolutely refuses to give me a replacement unless my phone rattles all the time without touching it or if I get a device other than the Galaxy Nexus.