The sound is only present when the aircon is on, at any setting. It sounds like something rotating that needs oiling or is vibrating around it's axis... or a pump that is noisy. I took it into (Croydon) Audi and they changed the unit with the fan in it for a new one (this isn't the fan that changes speed in the dash, it's the one under the bonnet). I still get the noise... the guy there suggested it might be a pump or something else, but they're confused. Any help as to what parts can be swapped out to get rid of the noise (which is only faint, and occures more when facing up-hill) I'd be very grateful... their workshop isn't busy, so I can get in quick with any work needed.