The design and development of modern internal combustion engines is marked by a reduction in exhaust gas emissions and increase in specific power and torque. This paper aims at the study of noise reductuion in4-stroke common rail direct injection engine at idle speed. Idle speed is basically a speed of engine when vehicle is not running i.e not in motion . Now a days , this siuation oftenly comes at redlights ,in traffic and in waiting while parked outside a business or residence etc. This paper presents a study about the effects of Fuel Injection Pressure on the combustion process. The end conclusion of the whole experiment is that reduction of noise takes place by optimizing the fuel pressure. In this experiment idle speed is 850 rpm. Then we set the fuel pressure at 200 bar at this pressure the noise is minimum which is 56 db. Further increase the fuel pressure the noise of the 4 cylinder CRDI diesel engine increases gradually at different stages like at 300 bar fuel pressure the noise of CRDI diesel engine is 60db.