A combustion-noise controlling method for a diesel engine includes: setting a target value for a vibration peak using one or more control variables needed for driving a vehicle, diagnosing a combustion noise of the engine based on the target value, and checking whether the diagnostic value for the combustion noise is higher than a reference value previously input to a controller. Combustion is performed depending on a result of the checking. A vibration signal of the engine is measured, and a vibration peak value depending on the measured vibration signal is obtained. A feedback control step includes obtaining a correction value by performing an arithmetic operation using the target value and the vibration peak value. A pilot fuel quantity is controlled according to the correction value.