Under the hood, check the main ground connections, including the one to the alternator. Also check the knock sensor itself. I don't know about your motor, but a good place to start looking for the knock sensor is "below" and to the left of the throttle body, near the throttle and cruise cables (I don't know if you have drive by wire or what). See that it's connected correctly and that it's not filthy.
If you are handy with a multimeter, disconnect the battery and see if you measure less than 400 ohms between the sensor wire and ground.
Next, reconnect the batter and turn the ignition to "on" BUT KEEP THE MOTOR OFF. You should measure less than 2V.