For the purpose of effective noise reduction, it is very important to understand the entire system of noise generation. So each noise source's contribution was analyzed in detail. Then the transfer paths and radiation surfaces of each noise source were analyzed. Combining all of the data, noise flow from sources to radiation surfaces became clear.Analysis results showed the importance of the main bearing structure. Several cylinder blocks with different main bearings were examined using a electro-hydraulic shaker. An actual engine was manufactured based on the shaker test results and about a 6dB(A) noise reduction was achieved.RECENTLY NOISE REDUCTION has become one of the primary goals of new engine development and an increasing amount of time and money is being spent to achieve this goal. For the purpose of effective noise reduction, it is very important to understand the entire system of noise generation. The noise generation system can be separated into three main phases. The first phase is the noise source, such as combustion, piston slap, or gear. The next phase is the transfer path of an excited force -- the piston, the connecting rod, the crank shaft, the cylinder head, etc. The last phase is the radiation surface, such as the cylinder block, the oil pan, the gear case, or the fly wheel.Many works have been written about noise source analysis [1, 2], transfer paths analysis [3, 4] and radiation surface analysis [5, 6]. But little research has been done into the entire system of noise generation. So a detailed analysis of the entire system of noise generation was planned and carried out.Noise source's contribution was determined using the coherence technique [7, 8], motoring method and special parts. For example, to ascertain the gear noise contribution, a timing belt was used instead of gears. Transfer paths were analyzed mainly by using a electro-hydraulic exciter. Finally the radiation surface contribution was measured using the acoustic intensity method with two microphones.Combining all of the above data, the contribution ratio of each noise source to total noise, its corresponding transfer paths and the radiation surface were made clear.Final results show that the largest force transfer path from sources to radiation surfaces is through the connecting rod to the crank shaft to the main bearing to the cylinder block. Modification of the connecting rod or the crank shaft is not easy because their inherent stiffness imposes severe limitations. Thus the main effort was directed towards modification of the main bearing structure. Five cylinder blocks with different main bearing structures were test manufactured. Then the noise reduction effect of each cylinder block was examined using a electro-hydraulic exciter. The best results were obtained from a engine with bearing beams. So an actual engine with bearing beams was manufactured and some other noise reduction methods were applied to the engine. The noise level of the modified engine was 84.3dB(A) and about a 6dB(A) noise reduction was achieved. This noise level is better than that of indirect injection diesel engines of similar size.