I'm fairly sure my car has the same issue. I have a '92 400E, but I think it's the same setup as your 300E. I've had this issue for a while, but just haven't got around to fixing it. Two weeks ago I had the car on jack stands while replacing the fuel filter and decided to take a look at it, nothing really looked too bad, but the bushings around the sway bar looked like they could be an issue. So, today I ordered those bushings at the dealer, and should receive them next week, once I install them I'll let you know what the outcome is.

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500