"Characteristic Bubbling" does make you wonder about the HG's. That's not normal!
As for the coolant disappearing, your radiator shouldn't be steaming. If you see steam, then you have spotted your leak. If the leak is at the top of the radiator it won't leak a lot so you won't see drips. But don't wait, replace it now before it really tears open on you as mine recently did!
When the head gaskets blew on mine, there was no subtle bubbling sound. The compression was being pushed out of the combustion chamber and into the coolant passages, forcing the coolant out of the engine and then the engine to QUICKLY overheat. The coolant was foamy, and was being pushed out the overflow bottle. Keep an eye out for these symptoms, or for milky looking oil if you suspect head gasket troubles.