Could also be a bearing or drive shaft.
Is this an automatic or stick?
If automatic, there's a way to disable the rear differential for diagnostics purposes.
I'm not positive whether your car still has this feature subies up to about 2000 have this fuse under the hood in a small box all by itself called "FWD".
If your impreza has that feature, putting a fuse* in that socked will turn the car into a front wheel drive car, disabling the rear differential.
See if it still makes the noise with that fuse out.
*pretty much any fuse. It just turns on a solenoid.