Sounds like a ship's horn?

Subject: Sounds like a ship's horn?

I live in the North Slope near Prospect Park, and every once in a while I hear what sounds like a distant ship horn. We're too far from Red Hook to hear the cruise ships, right? Sometimes I hear the noise as late as around 10pm, which I assume is too late for the ships anyway, right?
Any idea what such a noise could be if it isn't a ship?
My wondering started as mild curiosity, but now I really want to know.