Noisy Demon trunk lid?

Hi all,

For a long time now, I hear a loud noise in the trunk while driving, especially on bumpy roads. Like if an heavy thing was bouncing into the trunk.
It is very unpleasant
Despite my searches, I didn't find where this noise comes from.
It's not the spare tire, jack or tools, not the exhaust, not the gas tank, not the rear seat, not the package tray...

I suspect my trunk lid. I have the early flat model, the one without a rib in the middle.

The lid is in an excellent shape, but is it possible that the inner liner hits the outer steel sheet causing this noise..?

I will try driving with the lid removed, but wanted to know what you think about that?

Cheers... :)
François