^Keep trying until you talk to someone who will fix the problem. It seems to depend on the rep.
Mine's pretty loose as well... one of those things that's just annoying - makes the phone feel/sound cheaper. My back cover has a lot of give around the usb/hdmi ports as well. Even my "cheap" Moto Triumph felt solid and tight with the volume rocker, and everything else for that matter. One of those things that seems silly to ask for a replacement, but if it was offered I'd take it in a heartbeat...
You just need some duck tape. Problem solved no more rattling! Adds aesthetics, and shows you're a real man as well! Just make sure you don't use my advice when your screen breaks! ;-)
Mine does not seem to rattle at all. Maybe I just haven't used it enough yet to loosen it up.
Have you used the volume rocker a ton.
Maybe this is something that is inevitable to all down the road depending on use.