The inner panels are fastened by spring loaded clips...a previous owner may have determined that that section was removable and pried it at one time. That would have bent the clips sufficiently to allow the "play" that you recognize as "rattles".
One thing most don't tell you is that the top of the panel is held in place at the window lip by 4 or more anodized clips. Grab the armrest (where it is secure to the door panel and pull vertically, parallel to the door. Jostle gently side to side as you do this to loosen the window clips. It will seem that the panel will not move, then suddenly "pop" the whole assembly will come off. Then scurry around and find the clips that have jettisoned from the door...
When you turn the panel around, you can see the clips that hold the center panel in place. You can refresh the tension with a flat bladed screwdriver. Tap the center panel, ensuring that none of the clips rattle.
Install is the reverse of removal...but let's see how far you get first...I can give you more detail if you wish...
