During my scheduled 30K service, the tech noticed a buzzing sound. He thought it might be the Timing Belt Tensioner. He needed me to leave it overnight so he could hear it from a cold start. It in fact was the tensioner. He claims that if the tensioner fails nothing keeps the timing belt from slipping off and then we'll have a major malfunction. I should mention that I'm usually listening for something and never heard a "buzzing sound". Maybe it's hard to hear unless you've heard it before. I did notice that the engine sounded a little better and responded a little better after the service.
Subaru has been notifying all of their service departments regarding this issue. I should point out that I have a 2005. I'm not sure if this issue concerns any other years.
I'd recommend anyone who owns a WRX that is still in warranty, to call for an appointment and just say that you hear a buzzing sound (whether you do or not, just incase) and you think that it might be the tensioner. I'm at 31K and I'm glad the tech and the dealership did the right thing in taking care of this and not hoping I would pass the warranty period and have to pay for this myself.
My local dealer in Auburn, WA did a fantastic job. I highly recommend them. The Renton, WA. dealership 7 miles away, wanted $230 more for the same service. Shop around before you have the 30k service done. Mine was $328.96 out the door.
Note: Spark plugs don't need to be replaced until the 60K service on WRX because they are "titanium". Don't get fooled into paying for items not required. The Subaru site and certatin dealership sites show slightly different services done at the 30K. Even the Subaru manual is slightly different.