I just got my a/c compressor housing replaced - since new (2005) it would make quite a loud grinding noise when the a/c kicked in. It was extremely infrequent ie once every 6 months or so.
I did a trip up north to the Hunter Valley where it's much more humid and the frequency ramped up to 4-7 times a day - dealer repaired it under warranty without issue (even though it would not do it once I got back home)

I'm heading up to the hunter again shortly, so should be able to confirm if that fixed it