To be fair, the Bible didn't say noah warned anyone. God told him to build the ark, he built it, gathered the animals, then it rained. We shouldn't read more into the Bible than it says.
It is true that the Bible doesn't state that Noah warned anyone but I would guess that, like followers of Jesus today, it is logical to assume he warned others of the coming catastrophe. Since the Bible doesn't state this happened, I leave it as an educated guess.
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