Auto Air Conditioning & Heater Repair: AC Compressor grinding noise, faulty component, metal noise

Question
I have a 90 300zx with 198,000 miles on it, and the only A/C maintenance on it has been R134 conversion, and o-rings replaced. My A/C has been working intermittingly for a couple of months now, and it just stopped working all together the other day. I turned the A/C on with the hood up, and I can hear a metal on metal noise coming from the compressor. I know this because I turn the A/C off and the noise stops. Does that sound like the clutch or compressor internal? At this point in time with as many miles that I have, wouldn't it just make since to change out the compressor? FYI, I plan on doing this myself at the same time that I have to change the timing belt, water pump,belts..Etc.Of course I will have the system evacuated beforehand.

Answer
Without seeing and hearing this noise it could be the clutch or the compressor.

Of course I will have the system evacuated beforehand.  You FIRST remove the faulty component and the accumulator and the orfice tube. Then you FLUSH out the sytem using a flushing agent, Then you install the removed components, add the EXACT amount of refrigerant oil in the system, Then evacuate, Then charge with the EXACT amount of freon.