I think this is normal and the car was doing this for many years now but dont know what it is. You have to be quiet to hear it and have the radio off but when the car is idling I hear something turning on and off and it takes a very small load from the engine. Its sortive like a pump going on and off.....is their something on w126's that do that?

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99 C43
98 S420
99 C230K
01 C240