I had a similar door rattle problem. The 124's have the zebrano wood strip that runs across the middle top of door. Four 8mm bolts hold this piece on from the inside and at least one of these screws in all four of my doors had loosened causing an annoying rattle.
I removed all four door panels and tightened these screws down and car sounds much more taught. It's interesting that these screws weren't installed with a locking washer, allowing them to loosen over time.
Tap on those wood strips. If you hear a rattle or it just doesn't sound solid, then one or more of those screws is coming loose.
'91 300E, 214K miles