I have spent a lot of time chasing this noise down. I spent a lot of time playing around with the tailgate, but that wasn't the problem.
They say a picture is worth a thousand words. So here are 4000...
https://photos.app.goo.gl/baDHIUGdn3aO85U23
https://photos.app.goo.gl/GlZg1KZbVrStnwT33...
We just became a Mazda Family.
My 2014 CX-5, My sons 2014 Mazda 3 in the middle and my other son's 2014 CX-5 on the right.
Too bad my daughter=in-law wasn't there, she has a White 2014 Mazda 3.