Geartrain noise can be a significant contribution to the overall sound level of diesel engines. Some engine manufacturers employ isolation solutions such as sound deadening covers and foam panels to combat the problem, but these add cost. Little has been published on geartrain noise reduction, and public standards for diesel geartrain design and development are not available. This paper describes an experimental study of the relative influence of gear design parameters on the rattle noise of a diesel engine timing geartrain. The geartrains of several diesel engines were benchmarked to determine the noise reduction strategies employed. A total of three gear sets were designed and tested in a 3.3L four cylinder normally aspirated diesel engine. The experimentation quantified the influence of an anti backlash idler gear in reducing gear rattle noise, and revealed that a key path for gear rattle noise transmission is through an idler gear journal bearing shaft. A backlash investigation showed that increasing backlash increases rattle, with a greater penalty at lower speed. While definite conclusions could not be made, there was evidence that changing to a different idler gear material with increased internal damping properties had an effect in reducing gear rattle noise. Increasing the contact ratio in a helical gear set and increasing the idler gear journal bearing clearance allowed the anti-backlash gear to be eliminated while maintaining the same overall noise level. The gear rattle sound became audible, however.