there rally isn't anything behind there. The only thing i can think of is that the bottom part (speaker grill and flap) that you have your finger on is actually a seperate piece that is kinda melted on... tabs are melted causing it to attach. maybe it has shaken a little loose.
I have had all my doors apart and again there isn't anything there. You could pay a body shop to take your door off and apply something to the back side like a super glue of sorts. This may cost you up to a $100.
If you were local i'd do it for ya. it may cost you a beer.
good luck. let me know if you need any further help.
If i'd read this a week ago i could have taken pics of the other side of the door for you.
the dealership said there are some parts behind the panel that keep it from vibrating and my panel was missing one. they fixed the rattle...but now I have a new rattle on the same door! I am seriously considering dynamat right now.
on the back of my door panels i didn't use dynamat i put that on the metal door itself.. I actually went to the hardware store and purchase 3M rubberized coating... I sprayed the rubberized coating on the back of all my interior door panels.. really helped. I used about 3 cans. Almost a can a door.. i'd give it at least 2 good coats. Make sure you tape of the holes and the speaker grill as you don't want the stuff on the front of your panel.
The part they were referring to was probably one of the melted tabs that hold the bottom section on.. it seemed like it was rattling.