My 400E has the same issue, wheels been checked, 2nd set of tires, road-force balanced, aligned, suspension components are old but not defective, new steering damper, etc etc. I'm having my shop give it an exam in a couple weeks. I was down to thinking it was front bearings, but they don't think so, and there is no noise. Also gets worse as I drive (heat?). I'll point them toward the calipers, but for now, it's a mystery.

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former: 83 300D, 97 C230, 93 400E

current: 08 C300 Luxury , 92 500SL