I'd like to hear other's views too, as I haven't had this on my (only) 105, but in a transaxle car, a very distinct whine that can be stopped and started at will by going just in and out of the throttle (crossing the "neutral" point if you will) is normally the pinion shim being worn. I had this on the Milano/75 I bought used back in '92 and had the dealer replace the shim as a condition for the purchase. (This shim locates the fore-aft position of the pinion gear compared to the crown wheel.)
The implication being that if it is this, the pinion bearings may need to be replaced and/or re-shimmed at the correct distance, which is hardly a trivial job.
But I won't claim this is definitive however...
PS Some may say that the NS variant oil isn't right for a non-LSD diff, if that's what's on your car.
Alcala de Henares, Spain