Sounds like RHS Top Hat. It's a pain to have Subaru diagnose as it comes and goes, sometimes really obvious even just rolling the car, other times quiet for days. You're probably still under warranty, tell Subaru you think it's the drivers shock and top hat, I know they've replaced ALOT of them by now, so they should just pull it apart and have a look as it is now a known issue with the underdampened Bilsteins. Mine was one of the first, so it took a few trips to the dealer to get it to make the noise while actually there, I would think explaining the noise and the intermittency of it should be enough for them to know what it is by now.