My wife has a 2000 C 280 and we noticed a ticking sound from the engine bay. We had the MB tech stop at the house, he only lives 3 blocks away, before making a full fledged appt. The ticking was perfectly normal and has to do with the flow of fuel being meterd into the engine. He put his finger on the little pump and that was it. You could fell the pump working. However, it will tick at certain times, but this I was re-assured was normal. Perhaps this could be the same with your vehicle???

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Dale, R129
W202