I will bet it's something around the suspension, I am betting the bushings. And I don't think mazda gives a crap and you might still have bushings from a 2010 or earlier in your car. Go to the dealer if still under warranty while you still have time and be insistent. I have a 2010 and you would think they learned about the rear bushings from 2006 etc., but now they did not, the master mech drove the car and told me they were bad without me even mentioning. I originally came in for a bad leaky rear shock. They did both shocks and bushings as the mech could not believe how much noise was coming from the back end. This was at 20 k, now at 40 k everything still sound ok. Mazda makes a van you can get for $20 k, the cheapest kia Sedona is at least $25 k, there is lots of cost cutting involved and it's not a high quality car like a civic. My wife paid $24k for a civic 7 years ago and has not touched a thing, it drives like off the showroom. But she has 1/2 the room, no auto climate, no 17" rims, but engine and parts are built way smarter and easier to work on. I believe that our engines are 2001 technology and low compression, therefore give us crappy gas mileage. Contrary, I would rather have a low stress gas guzzler then the skyactive firework that nobody knows how durable it is, since it's so new. But we shall see. The new mazda3 manifold is a convoluted mess that will be really expensive to fix. What if their engines become these unstable rockets that blow up after a certain mileage? Have they really done enough testing or do they even care? I think the euro spec 13:1 is ridiculous. Audi and BMW had problems with blown high compression engines too. If they can't get the bushings right can they get a new engine right? Bushings are mazda specific, I think ford had their own, they look different from what I can see on a focus for example.