Diesel is a light oil distillate. Like others have sad when cold, diesels will either blow a puff of blue smoke or smoke white for a while due to moisture in the exhaust system. Obviously diesel ignites in a diesel engine because of the intense heat created from the super high compressed air. When the engine is stone cold not all of the fuel is going to burn completely and your engine will be slightly louder when cold because of the dramatic temperature rise "AKA" Combustion and the colder air.

I would also think that maybe the Crankcase ventilation system contributes in that you get a SLIGHT build up of oily vapors primarily from shutdown at normal temperature when you have vapors still collecting in your intake system. Let them sit and collect over night or a few hours and maybe they contribute slightly.