Strange foul/burnt smell coming from engine area on 2004 Mercedes-Benz C240 4MATIC

We recentely purchased a used 2004 Mercedes-Benz C240 4MATIC. Once in a while there is a strange foul/burnt smell from the engine area, while driving, that comes strongly than goes away, but we have not noticed any leaks underneath the vehicle or any other symptoms that would indicate a problem….. Does anyone have an idea what may cause this?

Asked by for the 2004 Mercedes-Benz C240 4MATIC