The noise of diesel engines operating at low idle is an important noise evaluation criterion in both commercial vehicles and passenger cars. At low idle, a quiet, pleasant noise is required. Therefore, any unusual noises that occur at low idle represent a serious problem, and the noises must be prevented. This paper presents the characteristics of the moan noise, which is an unusual noise that radiates from the oilpan at low idle, and the mechanism by which the moan noises are generated in a diesel engine. In addition, a means of preventing the moan noises is discussed.

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