I can tell you what I found on the inside of my door when I took the panel off that I figured was for this very reason. Someone in the past had glued some of those self adhesive felt pads, thick ones, to the partical board. It probably puts enough pressure on the panel to stop it from rattling.

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97 E300D 103,000 mi - Mobil 1 Delvac 5W40
87 E300D 145000 mi - Rotella 15W40
05 2500 Duramax 27,000 mi - Rotella 15W40
“You’ve got the using kind of religion, not the meeting house kind” - Rev. Pile