Speaking as someone who has had both a problem camera and a normal camera, I want to point out that there are two very different noise levels involved with the D800 rattle question.
The normal camera does, as everyone has noted, make a rattle if you shake it, and otherwise is not noticeable and not a problem. My current D800 does exactly that.
But with my first D800 the noise was exponentially louder - not in the same universe as my current one. Just gently tipping the camera from side to side - no shaking at all - would create a noise that sounded like screws were rolling around inside the flash housing. It was loud enough to be heard by anyone in the same room, not something you had to really try to hear. Something with at least one of the flash hinges was clearly defective, so I had the camera replaced with a new one.
For still photography I guess I could have ignored it, but occasionally I use my D800 to shoot video with the built-in mic, which sits right next to the flash. If I moved the camera enough to create the rattle it would be recorded very loud and clear, and for me that was unacceptable.
Bruce, if you have anything like my first one, I would send it back while you can, especially if you will shoot video. But if you have to shake the camera to hear it, then you probably what others have described and it is not a problem.