Noise & sound quality has gained equal importance as that of emissions and crash safety of the vehicles. With increased engine power to weight ratio, the challenges for NVH engineers has increased multifold. Passenger compartment comfort levels are getting affected largely due to lighter and powerful engines. Same time, there is pressure to reduce overall vehicle weight and cost. This impose constraints to NVH engineer in designing the body structure and sound package to reduce the effect of powertrain forces and airborne noise on passenger compartment. In addition to weight constraints, there is trend emerging to use two & three cylinder engines which need to perform on par with four cylinder engines. This has shown adverse effect on vehicle NVH performance due to wider low frequency unbalance forces. Unlike the traditional mechanical pump engines, today's engines are equipped with electronically controlled multi-injection combustion process, which is of immense benefit to automotive engineers in optimizing combustion process within constraints of noise, emissions and fuel efficiency. The present work discusses the process used for NVH refinement of common rail electronically controlled diesel engine. The effect of Common rail injection parameters on overall engine noise & sound qualities is thoroughly studied. Three common rail diesel engines are used for the study. The DOE (Design of Experiment) were followed to focus on critical combustion parameters which largely affect the noise. In order to separate the mechanical and combustion noise the physical measurements are conducted on engine dynamometers inside acoustic test cell. Engine irritants like timing gear rattle; knocking, combustion roughness and whine noise were identified and improved subsequently. The subjective - objective correlation parameter like annoyance index is also explored to assess the NVH improvement.