Could the trapdoor be the lid of the glove box? Maybe? I dunno.
I had a similar problem on mine. A very annoying sharp rattle on the passenger side.
With mine it was a little area of the inner door panel rubbing against the dashboard. (That vertical panel that is "at the end" of the dashboard). I rubbed some household soap on it and it was gone.
It could also be the dash pad that came a little loose and is now moving.
Such things are always hard to figure out, especially from a distance.
Best is to proceed by trial and error. Have a passenger hold the door slightly open when you drive. If the rattling stops, it's the door rubbing somewhere, if it doesn't stop.... well, at least you know what is NOT causing the problem.
Remove the dash pad and drive, and so on.
Narrow it down.
That's how I found the problem.
Good luck!