In Zürich, the worst offender is the outdated two cycle moped. Sadly, not much is being done about it because of the cult and retro status of these things.
People have loud motorbikes and cars because they want attention. It seems very fundamentally selfish.
I've spent a lot of time in the countryside where there is a motorway in the distance, and the general low rumble road noise seems to be mostly the tires etc. But then sometimes a loud motorbike revs and that really carries and intrudes.