Abstract: A generator support plenum for a engine-generator set for installation on a pickup truck in an RV or on a boat, for example, sits in or under or forms a part of an enclosure for the engine-generator set and defines cool air flow paths into the set for cooling the engine and generator and aspirating the engine. The plenum also defines a warm air outlet duct for escape of the cooling air. In the outlet duct a diffuser couples to the exhaust of the engine to mix hot exhaust with the much cooler escaping cooling air that has passed over the engine and generator. The diffuser eliminates the effect of the extreme hot exhaust gas and muffles the engine sound. Insulation coating the internal surfaces of the plenum further suppresses engine noise.