I'm still voting for a head gasket leak. As the aluminum head warms up, it will expand more than the steel head bolts and the cast iron block. Any small gap will tend to close up.

Early imports with aluminum heads had some problems with gaskets due to expansion. The engineers were used to cast iron and didn't compensate with bolt and gasket redesign.

It could be a loose bolt, you could retorque the bolts and probably seal it up. Some head gaskets do require a retorque after so many hours to take up initial crush.

I'm thinking that at some point you will blow the gasket by pulling out from a stop when the engine is still cold.

Also, are you losing coolant? If yes, then I really think its the gasket. And I really hope its not a crack.

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120