I can't reply to Ingin's thread, so I'll add an observation here, if I may.

My 2006 C180K does the same thing, as does a friend's. He doesn't have Comand, and mine did it both before and after Comand was installed. The sound is very faint but it's there all the time. It hasn't caused any problems in the nearly four years I've had the car.

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Paul Fisher
Perth, Western Australia
My little ego trip