S203 C320 - adjusting door locks/ clatter from passenger side

[eerola]

My 2002 C320 wagon has an extra clatter noise coming from the passenger side. The noise is very easy to hear, but it is not continuous. And it does not need any bumps to start. The car can be totally quiet when driving over speedbumps, but then the clatter starts when driving a virtually perfect road.

When sitting on the driver's seat the sound seems to come from the rear right door. When sitting on the backseat, it seems to come from closer to the front passenger door.

The noise sounds to me like the passenger side door locks would need some adjusting. But pushing/ pulling the door has no effect on the noise, which makes me think if it is the lock and striker after all... I have also checked underneath the car, but did not see any obvious source for the noise there. So: Good fresh ideas are welcome!

I have tried adjusting the door lock strikers, but they do not seem to allow as much moving as in W124 - for example. My plan was to move the striker clearly upwards and see if this eliminates the annoying clatter. But it does not move more than 2 mm or so. How do I adjust the doors in the 203 C-Class?