To determine the engine noise reduction methods, an engine noise research was conducted experimentally with a PCCI diesel engine. The engine employed in the experiments was a supercharged, single-cylinder DI diesel engine with a high pressure common rail fuel injection system. The engine noise was sampled by two microphones and the sampled engine noise was averaged and analyzed by an FFT sound analyzer. The engine was equipped with a pressure transducer and the combustion noise was calculated from the power spectrum of the FFT analysis of the in-cylinder pressure wave form and the cross power spectrum of the sound pressure of the engine noise. It is well known that the maximum pressure rise rate is the main parameter related to the engine noise. The PCCI engine was operated at a 1.0 MPa/°CA maximum pressure rise rate to eliminate the effects of the maximum pressure rise rate, and parameters which had the dominant effect on engine noise and combustion noise were determined. It was found that the combustion period and the maximum rise in the rate of heat release were the second and third most important parameters in the generated noise. The effects of the combustion period and the maximum rise in the rate of heat release on the noise frequency ranges were different. The combustion period is statistically closely related with the high frequency range of the noise, and the maximum rise in the heat release rate is clearly statistically related with the middle frequency range of the noise. Further, the mechanisms of the sound reduction by supercharging and EGR were investigated, and high load and low noise engine operation was shown to be possible with a combination of EGR and supercharging.