I've made mine own knock listening device.
$20 Bosch knock sensor (used at a Volvo junk yard, excuse me, salvage yard)
$10 cable with aligator clips and a minijack audio plug from Radio Shack
Plug the microphone (knock sensor) right into the computer.
I filter out all frequencys except 5-10 kHz with equalizer in sound software (I use a Mac specific program Final Vinyl but I've used other effectively--30 band equalizer, but it's overkill). Sound is processed and played through on the computer speakers or via head phones. Det sounds are very distinct as everyone knows.
It's just for tuning, but it works as well as TXS Tuner.