1) Possibly your gearbox bearing (1st). Mine has been noisy for months now.. although if extremely noisy I would change it but other wise next time you drop the gearbox (e.g clutch change) change it then.
2) If it is the above then u will hear the sound whilst ever the clutch isn't engaged.
Could be completely wrong! But thats what the sound is in my car. Although mine isn't so much as a whine but a bearing sound!