Intermittent noise on WRX

If it just started, it could be something as simple as a tank of bad gas (maybe the station you last went to had the wrong grade in their tank - it happens - the local news did a special on that out here one time, and it was alarming how many stations were selling regular as premium...) - just the same, I would bring it to the dealer and have them take a look (drop it off the night before, so that they can listen right after a cold start when they can look at it in the morning)