I found this while messing around on the SCT Pro Racer forums tonight. Sounds pretty interesting. I have an extra pair of knock sensors so I might give this a try just to see what it does. Pretty neat idea even if it appears that the author had a little too much time on their hands.
http://home.netcom.com/~bsundahl/knock/sound/KnockSounds.htm#Knock
http://home.netcom.com/~bsundahl/knock/sound/KnockSounds.htm#Knock