Boat Noises?

A reader left this question in the comments: A question for the blog: Do you hear a noise like a fog horn or a ship's horn on occasion? Is this noise from the Navy Yard, or from cruise ships coming and going from Red Hook, or from ordinary East River ship traffic?

I've never heard any boat noises personally. I don't think the Navy Yard is used for boats anymore, but I'm not positive about this. Anyone know the source?