For what it's worth, my wife's '03 Forester has this very same problem. It only happens when she puts it into reverse after it's been sitting long enough for the engine to get cold. By the time she backs out of the driveway and puts it in drive, it's gone. But it's still a bit jarring.
The brakes are 55,000 old and just about ready to be replaced, but the noise happens whether or not the brakes are applied. All the fluids (transmission, power steering) check out fine.
Anyone with any insight on this would immediately be annointed as a god/dess. My gut is that it's nothing and probably related to the brake assembly, which will be replaced soon. But in the back of my mind, it nags at me that it's something transmission-related.