As already mentioned, check to see if the oil filler cap is secure and sealing. Since you replaced the belts check to make sure they are tight, slipping belts don't sound like a honking noise but I don't know what horn you are referencing. Sound could be coming from the dipstick, or even from a leaking VCG. Could be very low power steering fluid but usually that sound doesn't go away, certainly not at full lock. You said you are positive it is a VCG leaking oil, have you checked if there is a power steering leak that could causing the smell?

If there is no vacuum at the oil fill hole it is sucking air somewhere else. Lots of places there could be a vacuum leak but you haven't mentioned a loss of performance or poor idle.

When the secondary air pump is bad it makes a grinding or whirring noise for the minute or so it is running when cold started. That would be any cold start not just under 20 degrees. It also causes a rough idle until the exhaust temp get hot enough for the engine to be in a closed loop mode.

That said, Mayo is key symptom associated with CCV system malfunctioning but with sound going away so quickly and no excessive vacuum at the oil fill or dipstick I don't think it is a sure thing.

I don't know how much mayo you are seeing and you mentioned it has been the same for a long time but I would be very concerned not knowing why it is there. There is always risk driving a vehicle when something is wrong and you don't know what. My guess is that a VCG leak and the honking are not connected and that the VCG would not be the weakest link and go out again, but that is entirely a guess. If the VCG is leaking you could put try sealing it externally well enough to see if the honking and leak are connected. Did you confirm the VCG is leaking by cleaning the engine and watch for new oil to appear?

I would do more troubleshooting before buying parts. If the honking goes away quickly, is only under 20 degrees and you aren't losing a pint a day I personally might take the risk driving it but I would do more troubleshooting to be as sure as possible I wouldn't cause much bigger problems by doing so.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas