Is it a metallic noise or more like plastic?
Here's a checklist of things that I'd look for, in order:
1. Bumper cover and any of the trim pieces on the rear doors, rocker panel, wheel wells, bumper, and around the liftgate opening and the license plate (and license plate frame if you have one).
2. Rear spoiler. Someone a while back had posted a service bulletin that Mazda has released for the rear spoiler making noise on some cars, but I don't recall under what circumstances the service bulletin applied. If it turns out that is your problem, then Mazda has a fix for it. I don't know if production cars come with this fix from the factory now or not. Mine was built in Aug. 2015 and does not have it, but it only rattles when I close the liftgate. Not sure if it's bad enough that they need to fix it, but I plan to mention it anyway at my next oil change.
3. Exhaust hangers. Make sure the muffler and exhaust tips aren't swinging around too much and banging into things. They should move a little, but there should be some obvious resistance there when you try. If there's little or no resistance in their movement, they aren't being supported properly.
4. Check the spare tire storage area. Might be something loose and bouncing around in there.
5. If it's none of those things, then get it checked out by the dealer ASAP. It could be any number of other things, ranging from the worst case scenario being something loose in the drivetrain which could be causing damage the longer you put it off, or best case scenario is something simple like a piece of trim you couldn't see or a loose heat shield on the exhaust somewhere. Regardless, if you're still under the factory warranty, any of these possibilities should be covered.