Got my new N5 (first one had rattling buttons and a dead pixel - the latter was what I actually got it replaced for), and this one seems pretty much perfect. No dead pixels that I've noticed yet, and the buttons are much tighter. Although I can still hear a faint "clicking" noise when I shake the phone. Could this be the autofocus mechanism I'm hearing?
Both of these phones were from the same batch, 311K.
You can actually tell by the way you shake your phone if it's the camera or the buttons.
If you shake the phone front to back (screen towards and away from you) you should not hear movement. If you do, the buttons are causing it.
If you shake the phone laterally or vertically (left/right or up/down) you will hear a faint clicking. This is from the camera.
If you can't really tell, just hold the buttons in place and shake the phone (laterally/vertically) and you will hear only the camera. If you shake it front to back and still hear clicking, your camera lens glass (attached to the case) is not adhered to the camera module (attached under the circuit board). Pinch the camera lens and screen gently and it will adhere.
Under the round camera less glass is a thin piece of sticky tape that ensures the camera module is centred and in place.
I know this because I had really loose buttons that would rattle when I was just texting or tapping the screen... Instead of doing an RMA I took the phone apart and modded it myself. I didn't want to end up with a new phone with dead pixels, light bleed, or anything like that. Buttons are now perfect.
