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RUMBLE is a low-frequency noise heard in high compression engines and accompanied by high rates of pressure rise. Thus, rumble indicates a rapid combustion process. This paper describes a method of studying this problem: by detection of flame fronts by ionization gaps. Results of experiments show that rumble is caused by deposit ignition in the end-gas region. These multiple ignitions occur after the normal flame front has progressed partially across the chamber. The authors conclude that this type of deposit ignition is generally a self-sustaining process, lasting until the deposits are burned out at a particular engine performance level. When the level is increased, the process repeats itself.