In general, customers prefer gasoline vehicle to diesel because of the noisy sound of diesel engine. So, many studies have been conducted to reduce the combustion noise of the diesel engine. According to the big progress of the combustion noise reduction technology, the mechanical noise form diesel engine is getting more noticeable. Therefore automakers should improve the mechanical noise as well as the combustion noise from diesel engine. The present study deals with the rattle noise from timing driving system of mid-size diesel engine, which is hard to separate from combustion noise. By using the angle domain analysis, the rattle noise source was defined as the scissors gear of the timing drive system. Although the scissors gear was used, unexpected rattle noise occurred from the gear during idle and low speed driving. From this study, we found that the pre-load of the scissors gear should be optimized to remove unintended rattle noise.