Really don't bother worrying about the head gasket, it's a steel MLS gasket that's bulletproof, I doubt it's that. But if you want to be put at ease, before they tear into your engine, have them do a compression test first, and I'm sure they will find nothing wrong there. If it's the coolant, then it's normal. It's always under pressure. Here's a way to "possibly" test for it. If your GEN4 is similar, you should have an upper radiator hose on the drivers side. If you can get to this easily, then squeeze it with both hands several times to slush the coolant around. If it makes the same similar noise as you do this or immediately after, then that's the noise and there is no need for concern. It could just be that you have some excessive air in there. If it bothers you so much, have them vacuum flush your antifreeze.