I checked my gear box oil today and found that the oil was abit frofey and that it was way over the full make on the dip.

Would this cause the whining??

Gearbox oils getting changed next week

 

unsure, but an oil like redline light weight shock proof can cover up any non major gearbox issues, it of course wont solve major issues

 

Overfilling will cause foaming as the gears rotating will mix the oil with the air.

Causes air bubbles in the oil which can cause lubrication/wear issues. Doubt it'll cause the noise unless its been like this for a very long time combined with regular hard use.

What model IMpreza/year do you have? (it helps to say you know we're not mind readers!! )

Another potential cause could a centre diff failure: As some models have a sealed viscous centre limited slip unit on the differential, these can fail causing gearbox oil to intermix with the viscous LSD fluid. The car will still appear to drive ok and still have AWD, but it will be more prone to wheelspin as the centre LSD unit can't transfer power properly

 

If your anywere nr carlisle mate pop it up to me I'll change your Gearbox oil with the correct stuff