There is a Ford A/C compressor on every new vehicle from this manufacturer today and also on many in the past. This is the component in the climate control system that provides the means for the air temperature to be lowered to its desired point. The A/C compressor is mounted on either the driver's or passenger's side of your Ford engine near the front. It is powered by the centrifugal force of the drive or serpentine belt system on your engine. The pulley in the front of the A/C compressor is always turning but not necessarily engaged. This engagement occurs when the magnetic clutch is electronically activated. The owner of the Ford will realize this when they hear a clicking sound and the clutch is turning. When there is a problem with the Ford A/C compressor, this component either will not engage when the system is turned on or the compressor itself is making a knocking noise when it is running. This knocking noise is due to excessive wear in the A/C compressor or a lack of oil in its system. This is a dry sump system where the oil is in the refrigerant and not in a sump. An injection of just refrigerant oil can be placed in the system to try and reduce the knocking sound. If that does not reduce the sound, then replacement of the compressor is warranted. On your Ford, the A/C compressor is not only used to cool the passenger compartment in the summer but in drying the air for the defroster in the winter time. So this can occur your air conditioning system must be functioning all year round and not just in the summer months.