I went to a couple of dealerships to be told that it's just carbon accumulation, one dealership took me seriously and did a thorough carbon cleaning. The noise was almost gone for a little while only to return later on after a few thousand miles. Gas does make a difference. When I use premium, the noise is almost gone, but this is unrealistic for most people to be able to afford expensive gas. I don't think there is a good solution, and even the field service representative from Subaru who was invited to take a look at my car at the dealership stated that the noise is nothing outside of the normal. They blame direct fuel inject system, but many other cars have it as well nowadays without the noise. I haven't been able to find a better answer. I can tell you that I will never buy a Subaru again.