A new dual sample rate technique has been developed and applied to measuring in-cylinder pressure and its oscillations due to autoignition. The harmonics of in-cylinder oscillations were found in good agreement with those obtained from the solutions of wave equation in a cylindrical container. The time of knock relative to spark timing was almost independent of the knock intensity, fuel octane number, and inlet air temperature. The knock intensity was almost constant up to the spark advance when about 100 % of the cycles were knocking, further spark advance resulted in higher knock intensity. The mass fraction of unburned fuel at the time of knock was about 10% and was independent of the frequency of the cycles knocking. These observations indicated that the phenomenon of knock is a single-site autoignition for intermittent knock and multi-site autoignition for severe knocking.