Does it buzz when you are stopped as well as moving? Does it buzz when the ignition is on but the engine not running? This might give you aclue as to whether it is an electrical buzz or a vibration.
Airbag is a possibility, but also perhaps the seat belt mechanism.
Buzzes and rattles can be quite hard to track down sometimes. They can even come from a different part of the car to where they sound like they are coming from.
B Max Titanium X 1.0 125 bhp in Magnetic with built in SatNav and rear sensors.