1. A method for detecting borderline spark knock in a spark ignition engine, comprising: positioning a microphone adjacent to a spark ignition engine; generating a microphone signal, wherein the microphone signal is indicative of audible engine sounds; processing the microphone signal to estimate the acoustic activity which takes place in the auditory system of a human in response to the audible engine sounds; generating an excitation signal indicative of the estimated acoustic activity; processing the excitation signal to identify audible spark knocks as a function of engine spark timing; generating a detection signal indicative of audible spark knocks; processing the detection signal to compare the rate of audible spark knocks to a desired rate corresponding to borderline knock; and adjusting the engine spark timing as a function of the rate of audible spark knocks until the rate of audible spark knocks converges to the desired rate.