I have an 02 WRX with 186k km's on it. Been doing this sound since 170k km's. When I had my motor done they did the clutch and resurfaced flywheel and it got louder. I also did Group N motor mounts and have all the bushings done in the back (outrigger, diff mount, diff lock) so I expected more sounds.
One of these days i'll pull the outrigger bushings and see if it improves, and possibly grab a new OEM flywheel as well.
Yours sounds a little worse from the description anyway, but I was only able to find one person to document a failure with a similar sound and I believe he had found the first bearing at the front of the trans was the culprit. (he also had shavings in his trans fluid which after 3 changes I have yet to have).
Search my name and you will find a video I made last yr.