You can certainly tell ours are diesels on startup on a cold Canadian morning, such as today (-24deg C..), if you forget to use a block heater, but other than that, most folk around here who have been passengers in our Golf TDi don't even realise it's an oil-burner.
I reckon that with my additive, and the best diesel I know of round these parts, my effective C/N is about 49/50.
Even my "agricultural" old TDi Discovery doesn't make anything like the clatter of most American diesels using my diesel mix, but, as I've said before here, it IS noisier here than it was in the UK, and that can only be down to the crap diesel, since NOTHING else has been altered. DIs are also inherently noisier than IDIs, remember.
Diesel fuel quality DOES make a difference in the decibel stakes!
- Richard