rattling in the back of my car?

Hi, I have a 2012 Mazda3, also in 2.5L (hatchback) which I purchased brand new. In around the 20,000-25,000 mile range (last summer) I started noticing a knocking sound coming from the right rear of the vehicle, usually under acceleration or certain left turns. The local dealership diagnosed it as (if I remember correctly) a loose bolt somewhere in the vicinity of the right rear sway bar mount. They tightened it and the sound has disappeared. I'd be glad to email you a scan of the printout they gave me showing where the problem was, if you'd like. I know it's not exactly what you're describing, but it could be a good starting point for where to look for the source of your issue.