Jon, what will you do when you have built a good knock sensor - how will you use it? Will you retard your ignition, alter the advance curve, use different fuel?
Also I'm assuming that you are concerned that your car may be suffering from pre-ignition that you can't hear? I can tell straight away if my engine is 'pinking' - is the 'knocking' that triggers the sensor different than the pinking we can hear.
From what I read knock frequencies happen in the 6kHz to 8kHz range so you need a "band pass" filter.
The time honoured 741 op amp will do this job well.
By the time you hear the pinking its too late and the damage, however small, is done. All modern cars and modified ECU equiped car are fitted with knock control. This will not only give you the best possible performance under different driving and atmospheric conditions but also protect your engine if, for example, you get a bad batch of fuel or run very lean with fuel starvation.
Hear's a trace from some of my data logging that shows the knock sensor data for each cylinder and how the ingition timming is modified per cylinder cycle.