The pressurization of the Freon is accomplished in the Cadillac A/C Compressor. This is the part of your A/ C system that is driven by the drive belt assembly on the front of your Cadillac. The drive belt is attached to the A/C clutch on the front of the A/C compressor. When the A/C unit is turned on the clutch engages which transfers the centrifugal motion to the A/C compressor. This will be known to occur by the clicking noise the clutch makes when it engages. When the clutch is not engaged the compressor is not in motion.
The Cadillac A/C compressor not only compresses Freon, but also refrigerant oil when it is in an operational status. This is a dry sump system on your Cadillac where the oil circulates throughout the entire system to lubricate the moving parts of the compressor. When the A/C compressor begins to make a knocking sound it is most likely low on this refrigerant oil. This oil can be injected into the system on its low pressure side just as Freon is added. It is not advisable to add more than just one or two cans of oil or it will adversely affect the A/C system from functioning properly.
If the A/ C compressor is still making a knocking noise after the addition of the oil, it will have to be replaced or it will seize up. The knocking noise is from the internal components of the compressor that have become excessively worn. If the A/C system on your Cadillac is still pressurized, then the repair can only be conducted by a trained and authorized technician by federal regulation. This regulation is in place so no unauthorized release of Freon into the atmosphere will occur.