I'd be hesitant to describe it as a "whine", regardless of my Thread Title. It's actually just like the noise you get when you blow across the top of an open new bottle of beer. "Hhooooooo". I misspoke calling it "whine".
It actually goes away with just a hair more throttle input, almost instantly, and really only makes this noise if you're barely on the throttle, or if you are decelerating in the 59-61 mph range.
It's really exactly what the TSB states, other than I'm 2 mph off (they say 63mph), and I'm sure there's some wiggle-room there. I've been listening to it for weeks now, and it really gets louder when I stick my head toward the back seat, hence the initial thought that it is the rear-diff. Once I saw the TSB, that almost sealed the deal, but I'm not that hard-up to stop looking elsewhere yet.
I've cycled the AC on to see if the clutch is going, and it's not. Where the AC idler pulley is concerned, that actually IS going 99% sure, but is a very different noise, and happens under very different driving conditions. I'm not worried about that, I'll just take the belt off when I have time, then fix for summer.
I personally doubt it's a center-diff or tranny issue, because it's "business as usual" when accelerating, decelerating, coasting, up-shifting, down, etc, unless I'm at the previously mentioned speed range, where the noise manifests itself.
I have pretty comprehensive records for this car and an amazing CarFax to match, and the person I bought it from was a 39 year old female Czech immigrant, who bought it brand new and did all scheduled maintenance @ Subaru. While that doesn't mean anything definitive, I doubt she did anything aftermarket to the car. I've done a pretty OCD search of the car, and found no traces of past mods at all.
EDIT: Thanks for the input guys.
