So it does not go away once the car is warm? I don't see how cold cranking could cause any gasket problems. You do have a correct antifreeze blend in the car, yes?

I would check things like the vacuum pump with a mechanics stethoscope to see if you can find the source.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine