The Hub

I have a Martin HPL acoustic electric that has a buzz when playing the open D string. I've looked closely at the bracing with an inspection mirror and checked the braces with a .004" feeler gauge for gaps. There are no easily identifiable loose braces. I checked the string ball ends to make sure they were secure against the bridge pins and I checked the electronics for anything that might be loose and potentially contributing to the buzz. Having done all of that and still being unable to identify the source of the buzz, I'm running out of ideas for things to check. So I thought I'd ask here to see if anyone has run into similar situations and has some suggestions.

Linked below is a 30 second-ish video demonstrating the buzzing sound. It tends to become more prominent as I move the guitar after playing the open D. In particular, the guitar seems to buzz more when the guitar is flat vs in playing position.

https://www.dropbox.com/scl/fi/vuo58if6hk71a9ssgbfvh/buzzy-d-string-on-martin-hpl-720.mov?rlkey=xswhbqkyj4ezg4y8q5d07awp5&st=t39595ty&dl=0