'12 WRX (10,490 miles) and I have noticed this just recently. This morning it happened around 30 minutes into my usual morning drive and it never went away. The handful of other times it would go away with the pumping of the clutch a few times in while in nutreal. I'm thinking maybe there is a tiny leak in the system somewhere that allows air in from time to time? I'm going to try and go over everything I can this Sunday and see what I can find.