I have a '99 Outback that I had new front axles put on about a month or so ago. This week a noticed a similar sound that is definitely in the drivetrain. My wife took the car the next day and called me on her cell phone to tell me that the car sounded like a jet engine. I checked the front diff fluid this morning and the the dip stick is bone dry. I am not sure if there is any fluid in the case at all but hope to check it out tomorrow.