I doubt the glass is loose, it's basically glued in. Perhaps the hatch frame is not closing tight enough and it's rattling. If so, as others have said, there are black rubber bumpers that can be adjusted by screwing them in/out with your fingers.I suspect it might be my hatch glass but I’m out of bumper to bumper warranty and not spending money to get it replaced!
Or, get in the back while someone drives the car and maybe you can hunt down the rattle location. Perhaps it's loose trim or a loose cargo cover.