Abstract: An engine noise reduction system includes a spring-biased valve (106) disposed in an engine air inlet (100). When the engine (104) is operating in a first mode that draws an increased amount of air through the inlet (100), vacuum pressure generated by the increased air flow overcomes the biasing force in the spring and forces the valve (106) open, maximizing air flow through the inlet. When the engine (104) operates in a second mode that requires less air, the biasing force overcomes the reduced vacuum pressure in the inlet (100), closing the valve (106) and thereby restricting the amount of air flowing through the inlet (100). The reduced airflow area changes the acoustic impedance for transmitting engine noise through the inlet (100).