An air duct assembly supplies air from an air cleaner housing to an engine throttle and includes a first duct connected to the air cleaner housing and a second duct connected to the engine air intake. Open ends of the first and second ducts are spaced from one another and the second duct has a flared bell mouth at its open end. The second duct includes a sleeve that defines an attenuation chamber. A flexible bellows overlies the first and second ducts and the sleeve, and extends across the space between the first and second ducts to provide an airtight connection therebetween and flex during relative motion between the air cleaner housing and the engine air intake. A hydrocarbon adsorbing material can be housed within the attenuation chamber.