My 2011 Mazda 3 auto hatch was going great until around October last year when it suddenly developed a knocking sound, which to me sounds like it is coming from the back passenger side of the car. The noise is intermittent and the speed or surface doesn't affect it. The mechanics at the dealership are having trouble finding the cause. It drives me crazy!
The knock could be caused by any number of things and the dealer is best placed to locate the problem. I would persist with the dealer, explain to him that you are frustrated and want it fixed, and keep on his back until it is.