The Dodge A/C compressor on your car is the driving force behind cooling down the passenger compartment. When turned on, the A/C compressor cycles on and off to meet the demands of the air conditioning system. When a Dodge owner is looking under the hood of their car with it running, the cycling of the A/C compressor can be noticed and heard. This is the clicking noise sound when the clutch pulley on the compressor engages. It is the component that compresses the refrigerant and sends it to the condenser in the front of your car. It is also the place the technician will monitor the high side pressure from. When the A/C compressor on your Dodge is operating and you hear a knocking sound coming from it, several things could be causes this noise. It is either low on refrigerant, oil, or both. The Dodge A/C compressor is a dry sump mechanism and the oil travels through the system lubricating it. If the level drops sufficiently, the compressor pump will prematurely wear and no longer able to perform its function. The changing of this part on your passenger cooling system can only be done by authorized personnel if the system still has pressure in it. This is mandated by government regulations so the used R12 or R134a in the system can be trapped and disposed of properly. If your system is open and no refrigerant present, then the owner of the car can replace the A/C compressor. If this is the case, then the owner of the Dodge should also replace the dryer on the system before a vacuum is pulled and new refrigerant placed in the lines.