I now have videos of my old Legacy making the same sound. They were on one of my backup drives.
http://youtube.com/watch?v=XV6mIeYRtTg
http://youtube.com/watch?v=UzY9XVYB3pw
http://youtube.com/watch?v=9btcz16bXXQ
It's only while accelerating (vanishes once gas pedal is released). And only if it's partial-throttle (about 60%-85%, when the pedal starts to give resistance). And only in a certain rev range (though this could simply be the engine drowning out the sound).