I recently had the slab springs replaced on my Mazda3. A couple month later I heard what appeared to be the very same noise so I returned to Performance Mazda. They looked over the car verified all was good and there were no loose parts. The technician/Mechanic search around and identified the back seats were not securely latched and the seat belts were on the wrong side. He latched them securely, rerouted the belts and the noise was eliminated.
When the service adviser came to get me, he presented me with a no charge invoice.
I felt this was a great example of value added service.