Info on boiling water

No, this isn’t cooking class for young, unmarried men. There was an interesting link today on Slashdot in their science column about how you can plunge your fingers into molten lead without burning.
Naturally, I was intrigued.
The full paper [pdf] explains why this is the case, and in building its explanation answered a question for me:
Why does a kettle coming to the boil start to make a roaring sound, and then get quiet when the boiling starts?