Diesel engine noise is classified into mechanical noise, flow dynamic noise and combustion noise. Among these, combustion noise level is higher than the others due to the high compression ratio of diesel combustion and auto ignition. The injected fuel is mixed with air in the ignition delay process, followed by simultaneous ignition of the premixed mixture. This process results in a rapid pressure rise, which is the main source of combustion noise. The amount of fuel burned during premixed combustion is mainly affected by the ignition delay. The exhaust gas recirculation (EGR) rate has an impact on ignition delay, and thus, it influences the combustion noise characteristics. Therefore, during the transient state, the combustion noise characteristics change as the EGR rate deviates from the target value.In this study, the effect of the EGR rate deviation during the transient state of the combustion noise is examined. A 1.6 liter diesel engine with a VGT was used for the experiment. The EGR rate was acquired by measuring temperatures using fast response thermocouples at the intercooler exit, the intake manifold, and the EGR cooler exit. The heat transfer model was adopted to improve the accuracy of the EGR measurement. As a result, the shorter ramp time in acceleration leads to the higher EGR rate, a longer ignition delay, greater RoHR peak value and higher combustion noise.