Gearbox noise...... advice please

Hi all
I have recently fitted what I thought was a decent gearbox. I have now discovered the car whines when driven. It does it in all gears but gets worse thru second and third. It also continues wen I put the clutch down and take it out of gear and the car is rolling in neutral. The noise has got worse since I put the box in a couple of week ago. Its definatley a gearbox whine and not a wheel bearing or alternator.
I do have a very slight gearbox oil leak which we have yet to locate. I also put a cheap fully synthetic 75w90 gearbox oil in just to check the box was ok before I fill it with decent oil. Could any of these be an issue before I pull the box back out. My mate suggested it could be the transfer box but do these ever go as Ive not heard of it before.
Any ideas please

Could be wrong but isn't it usually bearings that whine ? So could be the bearings on the input shaft? Or failing that it might not even be gearbox at all it could be clutch release bearing possibly? I'm not an expert by any measure but just some food for thought hope this helps dude

I would of said synchros .....but that would be a guess ....

You say it has a gearbox oil leak. How bad is the leak? ( has the gearbox been run dry) & burnt bearings
A whine will either be input shaft bearings, mainshaft bearings or diff bearings. Full strip to diagnose
Syncros cause selection issues & not noises.
I'd 100% be checking wheel bearings first.
I've had a noisey transfer box before. Had a good 4cm play up & down on diff bearing. Noise was worse on over run.

Could be transfer box as James mentions above, have come across more than one with worn bearings, same with layshaft bearings.
Very unlikely any of the used boxes are much good these days so all need looking at before fitting really.
I personally won't buy a standard box without pulling end cover as if the gear teeth are badly worn (majority are) they just a few spare parts at best.

mine has whined since I've had it, bob has had a listen as well, we suspect a bearing, still pulls fine, goes into gear fine, transfer box has been replaced, as has clutch bearing, been about 2 and a half years now

No James it has not run dry gonna have a look today and see where the leak is coming from so I let u know.
Thanks for the info peeps.

James.....It is bad on the over run aswell. I have a spare transfer box. So could try swapping them over.