Compressor should not be engaged in the off position, unless!!! the control circuitry does not know that the off postion has been selected. Chances are that Benzmac is correct in that the MAS relay is hanging. If the blower and the doors shut down when you hit off then it is probably the relay. If everything stays on, the the control center is probably to blame. Compressors get noisy for a number of reasons. Air in the system, low oil level, defective valves, overcharged system. Usually, if everything is in order, you should only hear a click as the clutch engages, the normal noise environment under the hood is normally loud enough to cover any operating noise from the compressor. If you are hearing the compressor, then you may have another issue that will need attention. I would start with that relay. It may be expensive, but it is cheaper than replacing a compressor and a system recharge

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Jeff L
1987 300e
1989 300e
1987 BMW 325