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God I can't believe there are 3 pages on this and nobody got the answer yet:
IT'S THE DIFFERENTIAL
I guarantee this is the answer. How do I know? My Subaru WRX has the exact sound you guys are describing. The reason comes from a firmly mounted rear subframe. The diff has a resonance right at about 60mph for me, and it got 10x louder when I installed subframe bushings. The resonance is anywhere in the 60-70mph range for all Subarus (newer subarus have slightly higher resonance points). It sounds like a loud, resonating pitch that fills the whole cabin of the car. The cabin acts like an amplifier unfortunately.
Unfortunately for me, it's very very loud. Nothing I can do, since I'm not pulling the bushings back out.
The way to mitigate this is twofold:
1. Buy high quality gear oil for the diff.
2. Add audio damping material to the car, around the rear subframe and the lower cabin.
I doubt anyone here will get a resolution from Subaru. This is not something easily fixed at the source.
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