My 74 240D had a mysterious knock in the front when I would hit a bump, or even turn into a parking lot. The problem was that the sub-frame mounts were fatigued. These are rubber shock mounts that cushion the suspension frame to the chassis and are easily overlooked. I suspect you are having the same problem.

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)