Ross, From the description it really has to be either front or rear crown and pinion contact. The centre diff cannot whine, in fact it's technically impossible for any diff to whine. It's the crownwheel and pinion that do, the diff gears are stationary unless turning a corner. There's no crownwheel and pinion in the centre diff. Your car has a pinion gear about half the size or one in a 3.54 Subaru diff. The 4.44 pinions wear out. Subaru did a heap under warranty and eventually went to the larger R180 as a result. Wheel bearings and diff side bearings don't whine as they are turning at less than a quarter the speed of the rest of the driveline. More like a rumble, often described as 'helicopter' noise. If you're certain it's coming from the front you could try different oil or adjust the tooth contact via the diff side adjusters but it's a bit hit and miss without pulling the 'box out and setting it with bearing blue.