Mazda 3 2011 Problems

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.

Mazda's service interval is every 6-months/10,000 km and while that might be more frequent than most other makes, servicing is what keeps our cars in tip-top shape and I wouldn't recommend you vary your service routine away from the Mazda recommendation. Doing that while the warranty is still in effect could become an issue if and when you had a warranty claim.

First check for anything that might have come loose under the car and knocking against another component. If there's nothing you can see then have the shock absorbers and the mountings checked.

Road noise from the rear is a problem many Mazda3 owners have complained about right from the launch of the car and there’s not a lot you can do. Fitting different tyres is the first thing that most people do, and like you have found, they make little if any difference. I would suggest you could try applying some noise deadening material to the panels inside the car, particularly in the rear, which seems to be the main noise generation area.