I am unsure whether we have the same problem of hollowness but its a try.
All doors in the 300se shuts with a beautiful "thud", except one. The rear passenger door shuts with a hollowness and we discovered why. 3 clips of the side-skirt is loose and thus we have the "echo" effect. Once the clips were replaced (plastic kinda cone-clips), the door was good again.
Yes - the inside panel had clips too ... which was also loose and replaced.
It all made sense.
Now the doors close happily ever after ...
126 tailed by a 203, 129 leading the pack.