It is more exactly like a smell of old lubricant oil heated by the hot engine block. Like I mentioned in another reply, my old 300 SD has a lot of blow back from an old engine and this blow back have a typical smell of burned oil.
When I stop on traffic lights with my A/C system running in ventilation only I get this typical smell inside the cabin. It is not as a smell of exhaust system but as a smell of heated lube oil.
I had no chance yet to verify what is around the A/C air filter.
Was my explanation enough?
When I asked about the carbon filter I do not mean that I want the filter to solve that problem but to learn better about the carbon activated filters.
Thanks

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Nyck - TX - USA