Annoying Rattle - Need ideas

[commbubba19]

2012 WRX Hatchback.

I have this weird flappy plastic sound coming from the rear of the car. I can only describe it as what a loose license plate sounds like if you tap on it. The sound only occurs when the body of the car shudders due to road conditions. It is definitely not suspension because if the bump or pavement condition is soaked up by the suspension, i don't hear it. But if its bad enough for body of the car to get the force, i hear the sound. Here's what i've checked:

Plastic panels around Hatch. Removed and insulated. Can tap on them and they are solid. All wiring secure.
Spare Tire carrier secure.
Exhaust secure. Can move it around and no contact with heat shield or any thing else.
Tapped on rear bumper and all is secure, no rattling.
Adjusted rubber stops on hatch with no effect.
Checked heat shield above muffler, secure no rattling.
Did rear seat fix with latches rattling via a bit of elec. tape.
Removed the retractible hatch cover. no change.

So basically when the car is parked, I can't find the noise or duplicate it to track it down. I can't think of anything else that could possibly be loose. The car is rock solid with no rattles except for this one and I just can't find it. Just wondering if anyone has any ideas that I might be missing.

[Anub1s]

Check the roof lining.

[kprugbyplaya]

Does the rattle creep up during idle? Also, does it rattle when you are at high gear and low speeds?

Cuz that happens to me. I was going to install Dynamat to cut of the rattle

[SlapChop]

I hear this same sound ( or similar ) on rough roads. I THINK its coming from inside where the seatbelt for the middle passenger comes out of the plastic. But my car is a sedan so the seatbelts are different for the middle passenger I'm guessing?

[scarmack]

I got rid of my rattle by getting window seal and placing it in between my rear deck and window. I have a sedan though. It fixed my rattle. Now I can finally enjoy my loud ass car haha