A bad rattle in the hatch area of a 718 CGTS

I have a pretty loud rattle, or more like a "clicking" friction noise coming from the rear hatch area of my 718 when I drive over any road imperfections. It's pretty constant on anything shy of glass-smooth tarmac. I believe it's coming inside from the cabin (not the engine compartment, nor the exterior)

Things I've tried:

I've removed the cargo cover. I've removed the oil/water trim caps. I've felt for lose trim, lose floor, etc. It's not the cargo lash-points. I've tried changing the hatchback door latch tension (the rubber feet on the door screw in/out). The side cargo holds are empty; the noise doesn't change when those are left open vs closed.

I can't identify the source.

Is this a common problem? Any tips to troubleshoot?