The Toyota A/C compressor is now a standard component on all cars and trucks being manufactured by this vehicle maker. It is part of the climate control system that not only changes the temperature in the passenger cabin, but also removes the moisture from the air when the defroster is operating. The A/C compressor draws in the low pressure refrigerant from the evaporator and compresses it. It then sends this high pressure refrigerant to the condenser in the front of your vehicle so it can be cooled. The ability of this air conditioning component to operate is from the centrifugal power it receives from the drive belt on the front of your Toyota engine. The drive belt supplies continuous motion to the A/C compressor, but until the A/C clutch on the front of it engages, the A/C compressor will not be in motion. The engaging of the clutch is the clicking noise the owner can hear when this passenger compartment cooling component is in action. When the A/C compressor is engaged and a knocking noise is emanating from it, the HVAC system could be low on refrigerant oil. This passenger compartment cooling component is of the dry sump variety. The oil that lubricates it is mixed with the refrigerant and travels through the entire system in your Toyota. An injection of refrigerant oil can be done in an attempt to lubricate the compressor. If the knocking noise persists, then the damage to the Toyota A/C compressor is too severe and it will have to be replaced or it will freeze up in a short time. Because of federal regulations, unless the refrigerant has already leaked out of your system on the Toyota, any repairs to this system and the removal of the old refrigerant can only be carried out by certified and authorized technicians. This is to help protect the ozone layer around the planet.