How many miles on your car? If the car is stopping correctly and not pulling from one side to the other, then I'd think your brakes are operating correctly. Follow-up on some of the other responses and if those don't get you to where you need to be answer-wise, you can try jacking up the car on that side.
CAUTION: Use jack stands, block your wheels and set your emergency brake if you're going to get under the car!
Rotate just that wheel while someone lightly depresses the brake pedal for you. While the wheel is braking, listen for the noise and see if you can feel any "catch" in the tire rotation. Rotate the tire quickly backwards and have your brake operator
stop the tire from turning. Then try rotating it forward and lightly brake to see if you can pick up the catch/sound you mentioned.
Once you know where the sound is coming from, then you'll know where to direct your next inspection steps.
Also, where are you located? If you're not able to resolve this from the inputs you get on here, you also might try posting in your "area" forum and ask if someone in your area with Subaru brake experience can give you a hand. Subaru folks do that for each other.