Clicking noise from rear of car

[NYC-SE]

My Yaris has developed a clicking noise coming from the rear of the car. It is a sharp metallic click that presents itself when driven over rough roads. Anything other than glass smooth asphalt can trigger it.

To me it sounds like a body noise, like a panel or a seam flexing. It does not sound like an "impact" noise. It sounds like it is in the car. But maybe it is something underneath and the sound is traveling thru the unibody and it just seems like it is inside.

At first I thought it might be something in the spare tire well, but I removed EVERYTHING and it was still there. License plate clicking on the hatch? Removed it, still there. The hatch latching mechanism? Drove around with the hatch open. Still there. I took the door card off the hatch and checked and tighten everything. Still there.

Crawled under the car and inspected the exhaust and heat shields. Looked OK. Grabbed the exhaust pipe and shook it like a man having a seizure. No clicking. Suspension looked OK. I could not see anything obvious. Took the wheels off and inspected the brakes, again nothing obvious or loose.

So I bought the car to the dealer as it is still under warranty for another month. They could hear the click, but they couldn't figure it out. They had it for 2 days and couldn't find anything. It's going back in next Tuesday and they're going to continue looking. Honestly I think they are stumped, as am I.

Any ideas?