I tried playing with the air con thing, but no luck. It is more of a whirl than a whine.
If I am on a small residential street with houses either side, the noise is more prominent as a more confined area and easier to hear. It is very obvious, so will be easy to show a technician I'd say. Tried recording it with no luck, but may be able to do somewhere quiet and enclosed.
I am going to hazard a guess that it is the turbo - which would mean something isn't right the way it sounds! It does it at all speeds/acceleration, as soon as you take your foot off the pedal. Normal driving through the gears does not result in any sound that I can notice.