Wheel bearings usually rumble (not whine) and don't change pitch when you change from accelleration to decelleration.

I suspect from your description that the likely cause is the rear crownwheel and pinion. These are inside the final drive assembly. (often called a rear diff although not technically correct)

The cheapest and possibly best solution is to replace the assembly with another from a similar model that doesn't whine. Beware that there are various ratios (sedan and hatch are often different) and various shaft configurations. Best to seek help from a workshop that know Subaru.