Noise, Vibration and Harshness (NVH) is a comprehensive issue of manufacturing quality that gives the purchaser a superficial feeling. The NVH performance of the vehicle has undoubtedly become an important factor and basis for consumers. When the internal combustion engine is working, it generates a strong noise thus making it necessary to reduce the vibration and noise. The noise of diesel engine is divided into acoustic and structural radiation noise according to different radiation paths. The structure radiated noise is mainly caused by the combustion noise and mechanical transmission noise, while mechanical transmission noise is mainly generated by the meshing gears in the diesel. This paper aims to solve the problem of a diesel engine abnormal gear whine noise under idling conditions with a combination of simulation and experiment. By gear micro-modification, the gear train whine noise is reduced by minimizing the PPTE of the gear train. The comparison of the simulation to benching testing shows a good correlation. It provides an optional solution to the modification proposal of the engine gear train, which uses the similar structure for the engine. Future work will dwell on a better understanding of the gear rattle noise, which will be discussed later.