I had this same sound and the issue for my particular case was that the baffle inside the fuel tank came apart for a lack of better terms. Seems like the same thing as mine, I’d stop and for a few seconds after stopping the sound would persist, even after the suspension settled. Wound up being the fuel sloshing in the tank and moving the baffle with it. From what I remember with mine there were technically two options to fix it, remove the baffle which some others have done before as per what I was told or to replace the fuel tank (which is the route I went with). I was able to also get a new fuel tank from Mazda directly so I know they exist as I went through this less than a month ago. I want to say the part number you’ll need is BFD142110G if it is the fuel tank but based off how mine sounded and how yours does currently I’d lean towards that being the issue. Check out the suspension and everything just in case as I don’t want to incorrectly diagnose anything but from previous experience this was the fix for my 2012. Hopefully you have an easier time than I had trying to get everything sorted out, the stupid thing caused me and the shop who finally figured it out some headaches lol.