IIRC there was/is a Technical Service Bulletin with regards to a 'whine' and it was suggested that dealers check the circumferences of each tyre as one possible solution.
again IIRC the circumference difference Subaru suggest should not exceed 1/4' , which is not a lot
so a small ammount can make the difference
The same size tyres from various brands can have a different circumference, which is why they suggest changing all 4 at the same time
Have to say though, I have changed axle pairs in the past without issue. Perhaps its the newage vehicles that are a little more suseptable to the problem?