Pull the door panel and check the 8mm screws that hold on the wood trim piece. Not sure about the 126, but the 124 has three of 'em that secures it, that have no locking washer behind them. These come loose over time and will cause an annoying little rattle.
Usually you can tell if this is the problem by knocking on the wood trim itself to see if it rattles.
'91 300E, 215K miles