Little metal cargo hooks are stowed so they don’t rattle? Did you do the test with the rear cargo cover removed?
Might try pulling the little panel on the inside on that side and see if there are any wires in the rear gate cord that are making the noise by bumping against the inside of the gate or the inside trim of the gate.
Another thing in that area is the fuel filler tube… so you have an overflow tube that is clamped onto the main filler tube, and then an evaporative emissions hose that is also clamped along there… those could rattle and maybe sound like they come from the hatch… although you’d hear that likely closer to the wheel well, not the rear hatch.
Taking lidksmy’s suggestion there - just for kicks you could remove the spare and all that stuff entirely from the spare tire well, and that would be a definitive “no” if the noise is still there….
Trailer hitch installed? Maybe the harness connector bumping around?
You could remove the passenger tail light and look to see if anything there is off. The little panel uncover the two small bolts that hold it in, and then it removes off of the little hooks that hold it in place by pulling it straight back from the rear of the car.
Have you checked the license tag on the back, or if you have a tag frame, make sure that’s secure? I had a rattle on an earlier outback and ended up using all 4 screws with some nylon spacers to secure the tag. That made some weird noises.
When you were riding in the back, did you press against the rear hatch at different spots to try and isolate the sound? See if something makes it stop? Helps identify loose trim, maybe an adjustment to the striker that the closer motor uses to shut the hatch, hatch rubbing on the bumper fascia, etc.
Hmmm. I don’t have a Gen6 to test… the closest I could come up with, while listening to the audio there, and tapping various things on the inside trim (lower sill trim, rear seat release, grocery hook, inside lower trim, fuel filler door, bumper fascia, strut rods, little covers for the backup lights)… the closest in pitch and something that would vibrate that quickly would be the little cover that is over the bolts for the rear tail light, or the light itself (maybe it’s a bit loose?) You can remove that little cover and leave it off, see if the noise goes away… it’s only held in by these goofy little plastic fasteners.
Another possibility is maybe a harness connector bumping on something… something with the fuel door maybe? The wire that locks it… isn’t there a manual release for the fuel door lock too near there?
Hard to tell how loud it would be inside the car to compare to anything though.