My first N900 rattled too. I had it replaced as it died a slow death as its gsm stopped working and got my new one 3 days ago. Guess what, it rattle too. I think it might be from a few sources as if I hold the or port and the volume button etc it stops slightly. No big deal. they are tough. I actually dropped this one from standing height on to concrete today (almost could not deal with having to look at it) and all that happened was a slight slight mark on the corner. I don't think the rattle is anything serious.