Traditionally, the knocking cylinder pressure trace has been characterized by an instant jump followed by a steadily decaying fluctuation. We found many cases where an increase in fluctuation amplitude in time could be observed. Thus, a coherent energy release triggered by the pressure wave typical for the initiation of knock was discovered. Possible mechanisms for the explanation of this phenomenon are discussed: First, the combined pressure and temperature effects on the flame propagation rate in the end-gas, second, a mechanism based on turbulence augmentation by compression. Third, a mechanism of acoustic or shock wave induced flame instability and fourth, a crevice based mechanism. It is shown that only the crevice mechanism is feasible under engine conditions. It is postulated that the very frequent “weak knock” is due to this phenomenon. Experimental evidence is presented for the existence of this new knock mechanism. The shock wave induced instability was investigated by inserting an artificial shock wave into the test engine at idle. This mechanism is shown to be capable of producing knock-like cylinder pressure traces without gradual amplification of the pressure waves.