So the noise is only there when the A/C clutch is engaged?
The A/C is working fine though? If it was low on lubricant, usually the AC compressor won't cycle properly - it will rapidly turn on and off, something like 10+ times a minute.
You could also perform a cool-down test to verify it is working properly:
1. Start with the engine off, and with the AC not having been run for a while (the evaporator has to be warm when the test first starts)
2. Turn the blower motor control to the Off position.
3. Start the engine.
4. Press and hold the A/C mode switch and
then turn the blower motor control to high speed.
Continue to hold the A/C button down until
the A/C status indicator begins flashing, Then,
release the A/C button.
It will take it a minute or two to run - if it was succesfull nothing will be flashing at the end.
If everything is working properly, and you are sure the noise is coming from the A/C compressor, I would be likely to live with it until it craps out completely. I can't think of anything more that you could try without removing the compressor.
Although, this site is such a great source of first-hand knowledge, perhaps someone else can offer some further diagnostic tips!