If you know what to listen for, you can hear knock. That's how we used to tune timing curves before computers came into the picture. And since I know what I'm listening for, I can hear knock on just about every modern engine too.
And bandook, if you can hear it then you should be using a higher octane fuel. Just because your engine hasn't blown up yet doesn't mean that you aren't contributing to the long-term damage of your car (i.e. blown head gasket etc.).
You can't hear knock "retard" because that is a computer function to adjust the timing, after the knock sensor hears knock.
