Reducing engine noise of passenger cars has always been a great concern to engine manufacturers. Of late, increasing attention has been paid to the noise developed during the idling operation of engines.There are various noises that can be produced in and around engines. In conducting an analysis of engine noise, the authors focused on the noise produced during the idling of gasoline engines that employ a 3-piece piston ring. The 3-piece oil ring has been identified as a source of noise depending on the ring geometry and design employed.The noise is often made near the top and bottom dead centers with peak amplitude around frequencies of 2 to 3 kHz. The noise varies with the tension of the 3-piece oil ring and the peripheral form of the side rail. It is more likely to take place when the peripheral sliding surface of the side rail, in contact with the cylinder bore, is of a high unit pressure design.The stick-slip phenomenon of the 3-piece oil ring was suspected to be the cause of the noise. The axial (vertical) movement of the side rail having high unit pressure within the groove of the oil ring during the sliding motion of the piston was examined using a motoring test machine. This test was undertaken to develop an understanding of the design parameters influencing the stick-slip phenomenon.It was confirmed that the side rail with higher unit pressure form moves more violently than that with lower unit pressure form. The stick-slip phenomenon of the side rail was thus identified as a possible cause of the noise. Accordingly, the low unit pressure peripheral form of the side rail combined with side clearance control is an effective solution of the noise problem during engine idling operation.