It's spooky that this thread has been resurrected today!
Today is the day I finally got around to taking mine in to the dealer's to investigate my whirring aircon. They found that it was low on gas, so they re-gassed it - and found that the noise went away.
Apparently the aircon compressor has a "scavenge" mode that it goes into when the gas is low. They said that in this mode it switches itself on and off very rapidly, and they're guessing that this could have been the source of the noise.
Whatever, on the way home it does seem to have cured the noise - and the aircon is now capable of blowing air cold enough to freeze the old danglies off!