Seems like adjusting the strike inwards about 1 to 1.5mm may have fixed it. Thanks for the advice Locry. Obviously as the door rubbers crush slightly with age the doors are not held as snuggly with the resulting movement causing the "nuck" sounds. The front doors now have to be shut more firmly.

Kramlavud, to perform the adjustment you need a T-40 torx driver for the two screws on each strike. It is the same arrangement on the W201 and W124. Only adjust them inward slightly and take care to maintain correct vertical adjustment otherwise it is difficult or impossible to shut the doors.

Now I only have to decide if I am going to go to the trouble of adjusting the hinges for the rear doors to move their leading edges in by the same amount. Since adjusting the trailing edge of the front doors inwards, the leading edges of the rear doors now sit about 1mm proud. From memory, the trailing edge of a panel should always be adjusted to sit flush with or slightly proud of leading edges of the next panel. This is presumably to prevent wind noise.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.