2001 Nissan Altima Knock Sensor

Engine Mechanical problem
2001 Nissan Altima 4 cyl Two Wheel Drive Automatic

I have a 2001 N Altama, I took it to the dealer to find out what the problem was with it and it is the knock sensor and a few other things which my son can fix for me but he doesn't know where on the car the knock sensor is, Could you please tell me where to locate the knock sensor on my car??