A GMC A/C compressor is the mechanical device that produces the flow of the refrigerant in this self contained system can perform its duties. As with all mechanical devices with moving parts, they can fail over the lifetime of your GMC vehicle. The A/C compressor is mounted on the passenger side front of the engine under the hood of the vehicle. It is supplied with centrifugal power by the drive belt system of the motor. When it begins to malfunction, the reduction in the air temperature in the passenger compartment will not occur as desired by the occupants. Audible confirmation of a malfunctioning GMC A/C compressor would be noticed when a knocking sound is heard when the clutch is engaged on the front of the compressor pulley. The closed loop air conditioning system not only contains refrigerant but also refrigerant oil. This is the oil that lubricates the A/C compressor. When the system is not fully charged or there is insufficient oil in this system the compressor's metal components begin to wear out and the metal components begin to move more than they were designed to causing this knocking sound. If the knocking noise just began or is not very loud on your GMC vehicle, the system might only need refrigerant oil injected into it. If the situation goes unaddressed for any period of time, the A/C compressor will need to be replaced because it will no longer be able to function are required by the system. The replacement of the A/C compressor along with all other components of the passenger cooling system on your GMC vehicle must be done by an authorized technician. This is to prevent the release of any refrigerant into the atmosphere as required by federal law. If the system is already open and no refrigerant present, the owner of the vehicle can then replace this component.