This paper reviews some previous findings about the combustion noise of diesel engines and compares them with the authors' findings. The reason why the noise of most turbocharged diesel engines is more sensitive to speed than load is explained. The findings of this research suggest that the frequency content of the combustion gas pressure up to about 300 Hz is related to the maximum cylinder pressure, Ṗmax. Between about 300 Hz and 2 kHz it is related to the maximum rate of cylinder pressure rise Ṗmax and above about 2 kHz it is related to both the magnitude and the duration of the second derivative of the cylinder pressure P̈. Pmax is more sensitive to load than speed. No correlation was found between Ṗmax and engine speed or load. The variation in the magnitude of P̈ is thought to be a random process, but the frequencies of the cylinder pressure fluctuations are closely related to the cavity resonance frequencies. For the engine under the same running condition, cases with higher values of Ṗmax and P̈ result in a higher engine noise level.