Also something i've just remembered from when i had an old "F" reg/1989 Subaru 4WD estate, if i had used the 4WD and not disengaged it properly, it created a strange vibration and humming noise - not quite like the buffeting from windows being open but similar enough to make me wonder.
Animals, all of them!
When I was working for IM in the '80s, the number of workshops who used to ask for one of us to go out to deal with that very thing was massive. To stop it doing that or, in severe cases to allow 4wd to be disengaged, we first had to drive the car a few yards in reverse to unwind the transmission, which was never meant for road use when in 4wd mode and had no centre diff. Then, if the customer's car had one end whose tyres were more worn than the pair at the other (inevitable in anything front wheel drive), we'd advise them to swap the tyres front to back as the manual suggested in large, red letters that they should do to prevent transmission wind up like this. At least we only did this once for each site that raised the issue, they wouldn't forget the bollockings or how to do this again and it was that rarest thing; a free fix!
However, later cars like that Legacy (all, afaIk) have a centre diff so that shouldn't be the cause in this case. It is possible that the resonance you feel is engine sourced, usually when the o/s camshaft slips a tooth or two on the belt. They're still safe engines even in 16 valve, single belt form though, so if this is the cause you'll not hurt it by driving to a specialist to have the valve timing checked against their factory waveform guides with an a/f probe and a 'scope.
