Recently I began to notice a loud humming / whining coming from the rear of the vehicle, especially at highway speeds. Over time it has become increasingly annoying to the point where I've gotten quite the headache after an extended trip. Initially I thought it was tires because they were older and slightly worn, but it turns out the sound was the wheel bearings failing.

After a bit of research I discovered Subaru is aware of the problem and issued a TSB #03-58-08. In that TSB, Subaru goes on to describe the exact problem I am having:

"Subaru of America, Inc. (SOA) discovered that there is a slight possibility that one or both of the vehicle’s rear wheel bearings may, over time, develop a noise condition that causes the bearing to produce a “whining” sound. This noise condition does not pose a safety problem and does not adversely affect the normal operation of the bearing. However if the vehicle experiences this condition, it should be corrected by replacing the affected bearing, which will eliminate the noise."

What's great is that Subaru has extended the standard vehicle warranty to 8 years / 100,000 miles for this problem. That means if you have a 2005 Legacy or Outback you are still within the coverage period as of the time of writing this (11/2011). However, I urge you to bring your car to your local Subaru dealer soon with a copy of the TSB:

http://www.fileden.com/files/2007/3/24/921208/03-58-08%20-%20Subaru%20Rear%20Wheel%20Bearing%20TSB.pdf

Hope this helps someone out there with the same problem!