if it goes away when you put your foot on the clutch pedal you might be lucky and it be something to do with the clutch, but it sounds like something in the box when its under load.

I overspeeded my xr3i on the m25 one day against a mate and later that night I could hear a faint whine under load which just kept getting worse and worse over the course of the next few weeks, sounded like I had a supercharger on the car, sounded ace but the box eventually locked up and spat the contents over someone's driveway.