Just heard it today whilst driving. Previously was only using the car on the weekends but becos the other car was at the paintshop, i've been using the 5 as the daily driver for the past week.
Today i heard the gearbox whine, most noticeable in 2nd, coasting down, throttle off. It whines also in first as well. Sounds crappy, like a electric motor or a dynamo spinning down. But only in-gear. Whine does not happen in neutral. I'm guessing something is wrong with the gearbox.
I've booked the car in for a full-on service + checks this saturday.
Is it something i should be immediately worried about? And will it incur some serious $$$$$$$$$$

h:
edit: ok, i got it, it sounds like one of the really old car sirens when going from a higher pitch to a lower pitch.
http://www.youtube.com/watch?v=r22aV3teNjM&feature=related