This is not "AS DESIGNED" because of a two stage button. It's a flaw that should have been caught and fixed. Think about how many other "two stage buttons" there are out there on phones that do not rattle. I have had three Lumia 900's. The first two I purchased brand spanking new around March-April 2013 and they did not rattle. However, one of them was left in the rain and got destroyed. So I bought the third and it DOES have the rattle.
Note to others out there. Be careful when you purchase a "New Other" unit because it's entirely possible that it was returned with this problem or another and consequently the "new other" status. I think that's what happened to me on this one. I had already paid ~$225 apiece for the first two so I wanted to save money and got a "new other" for the third. It rattles annoyingly loud. It defeats the point of having a "silent" or "vibrate" function because everybody in the room hears your crappy phone rattling.