This article presents an investigation of the sources combustion-generated noise and its measurement in HCCI engines. Two cylinder-pressure derived parameters, the Combustion Noise Level (CNL) and the Ringing Intensity (RI), that are commonly used to establish limits of acceptable operation are compared along with spectral analyses of the pressure traces. This study focuses on explaining the differences between these two parameters and on investigating the sensitivity of the CNL to the ringing/knock phenomenon, to which the human ear is quite sensitive. Then, the effects of independently varying engine operating conditions such as fueling rate, boost pressure, and speed on both the CNL and RI are studied.Results show that the CNL is not significantly affected by the high-frequency components related to the ringing/knock phenomenon. In contrast, CNL is found to be sensitive to increasing energy in the 0.4 to 2.0 kHz frequency range generated by the combustion-induced uniform pressure rise. Parametric investigations emphasize the fact that the RI and the CNL are designed to provide two distinctly different but complementary measurements. RI is designed to correlate with the sound produced by the resonating wave leading to knock. Hence, an RI limit appears to be a better criterion for avoiding knocking combustion and its detrimental effects (irritating noise, loss of thermal efficiency, engine damage). However, RI is not appropriate for estimating the overall noise of the combustion event since it does not consider the noise generated by the combustion-induced uniform pressure rise. In contrast, the CNL provides a valuable metric for the overall loudness of the combustion event. However, the CNL is not sensitive to knock-induced noise, so the irritating noise of knock is not accounted for in the CNL, and the CNL is not appropriate for estimating the onset of knock.