The sigil of House Greyjoy is the mighty kraken, but their sound is that of the sea at storm, in an endless rising and dying of the waves.
Just like a storm, one cannot see the Ironborn approach until it is too late; although sometimes it can be felt in other ways. Balon Greyjoy once styled himself as King of the Iron Islands and rose against the Iron Throne but his rebellion was swiftly brought to an end. Those who have lived through the rebellion said that at sea one could hear the waves roaring as if singing the sea shanties of the Ironborn. One can only hope they won’t rise again.