The exhaust noise of a two-cylinder four-stroke diesel engine was experimentally investigated in order to examine the predictability of the exhaust noise to be reduced by a feedforward active control system. Special attention has been paid to the low frequency characteristics of the exhaust noise as low frequency noise is difficult to reduce with a conventional muffler but effective to control by active control technique. The periodicity of the exhaust noise was examined with the ensemble-average and ensemble-standard deviation of the exhaust pressure signals. The results showed that the exhaust noise investigated was basically quasi-periodic and its variation between cycles was acceptable with an adaptive control algorithm. The properties of the exhaust pressure in low frequency domain were analyzed with the spectra of the exhaust noise pressure. A method to estimate the instantaneous exhaust noise pressure proposed by the author in 1996 was applied to this diesel engine by estimating a limited number N of low frequency harmonics in terms of engine speed and load. This method matches the active control system by its fast response. The predicted and experimental results agree well and show that the prediction method is valid and the exhaust noise pressure is predictable with this method.