That passsage starts at an exh port. You may not have flow, but the head side gets extremly hot. I've had bubbling int here before too. Unless you experience a loss of coolant I wouldn;t sweat it. All kinds of junk goes down in that area.
I'd guess just some blow-by pushing a little oil into it over time.
Unless you're overheating, as everyone else see's it, no biggy.
Cool it off and re-torque the bolts.