It seems to trigger mostly if I'm driving on a road with corners or small dips - which initially made me wonder if it was simply air getting trapped somewhere and causing a whistle - the sound is coming from the front, but it's hard to tell if it's coming from under the bonnet or through the air vents. Doesn't sound like it's coming from the gearbox, but I shall try and pinpoint it next time it happens - I never know when it will do it and sometimes it doesn't do it at all.