AE86ers.org posted a video of an AE86 doing Central Circuit in Japan:

The first thing I thought was that the driver of the car really needs a new ring and pinion! (aka crownwheel) It sounds the same as on my old Trueno but then at least four or five times as worse! You can hear the diff whining from about 50 km/h till 140 or 150 km/h, that is if I remember the pitch of the sound correct and presume the owner still got a 4.3 ring and pinion! ;)

The whining diff is a very well known issue of the AE86: its design was never meant for drifting, so if you put too much stress on it the crush sleeve will deform. Only a small deformation on the crush sleeve will cause an imbalance on the ring and pinion and wear down the teeth of the ring much faster than normal. So in case this happened to your hachi stressing the ring will only wear it down more and the whining noise will get worse! To prevent this TRD created a solid spacer which can not deform.

Another possibility for this amount of noise is that the AE86 is driving on the circuit with uniball 4 links. These will return all vibrations of the axle back to the chassis. Correct me if I’m wrong, but with uniballs you would hear much more noise and feedback from the road than this.

Also you may think the noise is coming from a straight cut gearbox. This isn’t the case as then the whining noise would change pitch when shifting up or down. Here the pitch is constant and continues with the same pitch after a gear shift.

So for all those people out there wondering what a whining diff sounds like on an AE86: this is it! If you hear it this much you really need to think about getting a new ring and pinion.