Abstract: Abstract: Dieselengine noise has a bad influence on physical and mental health of agricultural machinery operators. In this paper, we aimed at eliminating the abnormal rattling noise of a diesel engine running at idle speed condition by simulation and experiment. Firstly, on engine test bench in a semi-anechoic room, the rattling noise was identified through sound intensity and sound pressure measurement, the nine point sound pressure results showed that the rattling noise was more obvious in front of the engine, and the sound intensity of the engine front part showed that the main part of rattling noise was generated from the timing gear cover in front of intake camshaft and exhaust camshaft. In order to eliminating the abnormal rattling noise, some improvements were also attempted, such as using acoustic shield over intake camshaft and exhaust camshaft, improving the accuracy of timing gears through gear grinding, optimizing the parameters of valve-chain hydraulic tension device, replacing new timing chain, but the experiments showed all these improvements had no effect in noise reduction. Through analyzing of the previous experimental results, a hypothesis was proposed that the idle abnormal noise might be caused by the knock between the intake cam gear and exhaust cam gear, thus replacing intake cam normal gear by anti-backlash gear might eliminating the gear knock, and the diesel abnormal idle noise might be solved. Secondly, in order to identify if the gears knock really happened, multi-body dynamic simulation was performed. By using Hypermesh and Abaqus software, the FEM (finite element modeling) model of the engine including crankshaft, cylinder block, cylinder head and other engine components were built, and the model tests of each part were performed. The results showed that the frequency difference between experiment and simulation was within 10%, the accuracy of the FEM model was acceptable. By using Excite software, the multi-body dynamics model including the gear of the intake camshaft were built separately, and the multi-body dynamics model of the valve timing system and the vibration model of the engine were also built. During the vibration simulation, the exciting forces including cylinder pressure, valve timing system, valve exciting force, and piston slap force were considered. The engage force and vibration velocity with/without using anti-backlash gear were also compared. The valve timing system simulation results showed that when diesel engine with normal gear was running at idle speed, the gears reversed slap was occurred between the intake cam gear and exhaust cam gear.The moment when reversed slap of timing gear occurred, the contract force on the work side was eliminated, and the contract force on the backlash side appeared.So the idle speed abnormal rattling noise was excited by the reversed slap between intake camshaft gear and exhaust camshaft gear was validated. Anti-backlash gear can eliminate the reversed slap of valve timing gear, the vibration simulation results also showed that the vibration velocity of gear chamber cover and cylinder head cover was 7 dB lower in the frequency range of 1 000 Hz to 2 500 Hz when anti-backlash gear was used, especially at 1 600 Hz, the vibration amplitude was reduced more obviously. Finally, the diesel engine with/without anti-backlash gear was tested on bench in a semi-anechoic room. The engine bench vibration test results showed that the idling vibration acceleration of gear chamber cover was reduced from 120 m/s2 to 50 m/s2 and the acceleration peaks were reduced from 8 to 4, the moments of disappeared acceleration peaks while using anti-backlash gear met the disappearing moments of the engage force on the backlash side which was calculated by simulation exactly. The 1m SPL and sound quality were measured by engineering nine points method and KEMAR(knowles electronics manikin for acoustic)artificial head, and the noise test results showed: when anti-backlash gear was used, the abnormal rattling noise disappeared and the 1m SPL in front of engine was 5-9 dB(A) reduced, especially in the high frequency range. The KEMAR artificial head test results also showed that the sound quality of the diesel engine was improved obviously. In conclusion, anti-backlash gear can be used in other noisy mechanical transmission parts.