There are 4 fog horns that need to be hit in close intervals of each other in an exact order.
This is the order:
1: On the ground entrance into the lighthouse, take a right and go down the hill to the water. Behind some rocks on the left when looking at the water is where it will be.
2: Take the slide down the lighthouse and into the Speed Cola tunnel, and when you exit the tunnel take an immediate right.
3: On the ground entrance into the lighthouse, take a right and go up the hill to the bridge. To the right of the bridge is where it will be.
4: Take the slide down the lighthouse and into the Speed Cola tunnel, and then go straight out and wade through the water until you get to the rock on the right. There will be a slight bend and it will be right behind that rock.