My N900 started rattling today exactly as some of you have reported. At first thought I was blaming the accelerometer as well, thinking during the past couple of calls it must have gotten in a loose state. I tried calling my phone a few times and it had a placebo effect of changing the rattle a bit... So I thought I was on to something. Knowing I could just ship it back to Amazon I was still a bit miffed that the two week old phone had developed a cripplingly annoying rattle. I found this thread and although I had cards in both the SIM and MicroSD slots, I thought, "Maybe my fasteners have come loose." I opened up the N900, but sure enough they were in place and only producing the slightest amount of rattle (that I doubt is even audible or noticeable through the back cover). "Crap," I thought... Then I noticed something. While open like this my rattle was no longer happening. I had a hunch... Battery?

Sure enough, I put the battery back in and it can easily slide side to side. Also, if I tipped the phone upside down, the battery would just fall out. I took a small screwdriver and bent the two metal clips that hold the battery in place further in. After a few adjustments I got it so it was a pretty good fit. Snapped the back cover back on, shook it like a madman, and... No rattle.

So, if you guys are having any issues still, and it isn't coming from your card holders, chances are it's your battery sliding around back there. I may end up wrapping the battery in a piece of paper or something similar to make it fit a bit better if the metal clips end up going soft on me again (I really don't know how or why it happened this time, to be honest), but for now everything seems to be in order.