I had a loose screw rolling around in my head....liner.
I thought about bringing it to the SC, but the wait times were crazy and the sound was so annoying that I just decided to fix it myself.
First, make sure the sound is actually coming from the headliner. I sat in the back seat while my wife carefully veered side to side on an empty road. I could clearly hear something metallic rolling around above my head.
Next, carefully pry the headliner down using some plastic trim removal tools. In my case, I needed to wedge a few of them in strategic places just above the rear seat before I finally had enough room to get my fingers between the headliner and the center beam to pull it loose. You may need to unplug a few wiring harnesses to lower the headliner enough to get your hands in there. Give the lowered headliner a few taps to reveal the culprit, then use any method you can to remove the item. I found success using some scotch tape at the end of a screwdriver. If you have something flexible with a magnet at the tip, that would be ideal. Be careful, as there's plenty of thin sheetmetal inside the roof structure that can cut up your hands. Ask me how I know.
When done, plug all the harnesses back in, then align and pop the headliner back into place.
Best of luck!