Interior sound levels of general aviation aircraft are reviewed and compared with corresponding levels in commercial aircraft, buses, automobiles and passenger trains. Noise sources are classified into three groups -- propulsion system, external flow and on-board equipment -- and their frequency characteristics are described. These characteristics include both broadband and discrete frequency components. The main transmission paths associated with the different sources are identified in a general manner. Potential methods for reducing interior noise levels are described with emphasis being placed on two approaches with wide application. One approach is the fuselage sidewall treatment and the other is the use of vibration isolation mounts for the engines. The discussion demonstrates the difficulties involved in achieving large noise reductions at the low frequencies associated with propeller noise and engine out-of-balance forces.