Series 3 gearbox whine

Hi,
Just changed the gearbox and clutch , however when in idle , out of gear ,I get a very slight whine from the box , which disappears when I engage the clutch , any thoughts ? . Could it be just a noisy box ?, it was second hand , could it be alignment ? just want to canvas a few ideas before I do anything .
I think I've answered my own question really , it could be a multitude of thinks , however any input much appreciated .
Cheers
Steve

If it doesn't whine when it's moving then I'd say clutch release bearing.
Otherwise bearings in the box.
If it fitted on ok then the alignment should be correct.