The valve(s) Tom mentions are the intake and exhaust valves for each cylinder. Replacing the valves would involve pulling the cylinder head and inspecting the condition of the valves and valve seats ... relativley expensive and likely not a job you'd want to tackle. Before pulling the head you need to diagnose the root cause of the engine vibration. Have your mechanic check the engine performance when the engine is hot and vibrating. If he's unsure then get a second opinion. You want to avoid doing unnecessary and expensive work. Better to pay for a correct diagnosis than replace parts haphazardly. Consider taking your car to a local MB dealer for diagnosis if you're uncomfortable with the independents' recommendations.

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Fred Hoelzle