I have noticed that each time my air conditioner goes off (set on automatic at 78 degrees) there is one knocking sound. What is this? And why is happening?

If the noise is outside it could be bad mounts on the bottom of the compressor or a broken spring inside the compressor. Both types of repair are not user friendly and the broken spring inside the compressor will be very expensive to repair because you have to replace the compressor.

If the knocking sound if from the inside unit it is probably the blower motor.