I think you're right that it's the glass, I narrowed down my rattle to the glass. Pushing down heavy on it dampens the rattle a little. Here's another owner that had the rattle fixed at the dealer when they completely replaced the rear hatch glass: Lose Metal Rattle Noise from Hatch. This seems excessive for such a stupid rattle though. And I'm out of basic warranty and not sure if I can get this covered by GM.Update: still have the metallic click. Lowing the rear passenger seat doesn’t help. I’ve removed almost all of the plastic trim from the hatch, even the stoppers on the hatch and it’s still there. It has gotten better somehow and is now even harder to replicate. I still think it’s the glass, I shoved shims in between the glass and metal hatch but it was still there. Seems so have lessened after that but it’s hard to say what made it marginally better.
P.S. for other commenters: it's not the hatch stoppers, this rattle is present even when the hatch is open and you bang on the hatch. It's coming from the driver's side rear part of the window.