03 WRX Bug eye came in for a transfer case bearing replacement. The bearing on the transfer case is a common part that wears over time. With any typical bearing noise you can usually source the problem quick and easily. To do this, you might want to have access to a lift, but further more just get the car on the lift securely, then have some one sit in the driver seat and in 5th gear or 6th gear (you want to use your longest gear) let them bring the car up to speed to where you heard the noise. Once the car is up to speed listen carefully and just follow the sound, it’s the bearing that holds the output shaft where it meets the rear drive shaft.
Usually I would also recommend checking the center diff. as well since the car is on the lift and you are generally in the same area as well so it’s never a bad idea to take a quick look. Because if you look at it this way your car has 130k+ miles on it.. checking to see if the snap ring in the center diff has popped out would save you time and money. usually a snap-ring that pops out is an indication of a broken center diff.