Take it to the dealer to have them look at it and diagnose. And don't let them tell you it's supposed to be that way and you have to live with it. Someone else here had that issue with a low mileage 5th Gen and it ended up being something in the rear differential that wasn't clearanced correctly when the diff was assembled.
It has always had it since I bought it but that was two weeks ago. I got the car with 5,500 on it and now has 6000. I'm pretty sure its from the rear diff or drive shaft and not a wheel bearing. I've heard many wheel bearings. The noise I hear to me sounds like a gear whine in the diff. I've heard it on many BMW's but the customers never complain about it and I didn't know if its normal for Subaru to make that noise to. It drives me up a wall. I'm going to take it to the dealer. Will update with results. Thank you for the replies.