What I mean is if you play a sine wave on top of it you can match it as though you were humming the same note as the engine. The exhaust on a real engine will "speak" at how ever many cycles per second (Hz, the frequency) the exhaust pulses dictate. A 4 stroke 6 cylinder engine has 6 exhaust pulses every 2 engine revolutions, or 3 per second.
The main pitch you hear should be:
Frequency = EngineRPM / 60 * NumberOfCylinders / n
n = 2 for a 4 stroke engine, or 1 for a 2 stroke.
I.e., taking the FZR 4 stroke for example with 6 cylinders at 7000 rpm:
Frequency = 7000 / 60 * 6 / 2
Frequency = 350 Hz
If I match a recording of this to a sine wave I get about 115Hz, about 1/3 of the right value.
Granted, if you have more than 1 cylinder firing at the same time, then that changes things :P