The Pontiac A/C compressor functions to help keep the passenger compartment at a desired temperature. This is the one component on your vehicle that will actually draw away from the performance of the engine when in operation; but for most drivers this is acceptable. The A/C compressor is mounted on the top side area on the front of the engine. It receives centrifugal power from the drive belt system on a continuous basis. Just like a refrigerator cycles on and off, so does the A/C compressor on your Pontiac. The energy from the belt is harnessed when the A/C clutch, located on the front of the Pontiac A/C compressor, engages. This is the only time the compressor is in motion compressing the refrigerant in your air conditioning system. When it is engaged, an inspector should listen to the compressor as it rotates. If a knocking sound is emanating from the compressor, it might be low in oil. The A/C compressor is a dry sump type of device or there is no reservoir the internal parts receive lubrication from. In an air conditioning system the oil is mixed with the refrigerant and lubricates the moving components similar to how a two stroke engine receives its lubrication. When additional oil is necessary to eliminate the knocking sound on your Pontiac, it can only be injected into the system on the low pressure side of it. There is only so much refrigerant oil that can be injected into the air conditioning system before it begins to affect its ability to cool down the passenger compartment. If your A/C compressor is still knocking after the first can of oil is injected, then the owner might have to be prepared for the replacement of this temperature control device on their Pontiac.