Blackbeard's Ghost!
JohnnyP: Some things come to mind. The first is the centerboard, as you've suspected. As the boat rocks from port to starboard the centerboard may be free to thump the inside of the trunk as it moves on the pivot pin. You should be able to verify this by opening the inspection ports to access the pin. Placing your hand on the pin, you should feel the thump as the noise appears. This condition could occur even if the board is up. Another noise could be caused by play in the gudgeons supporting the rudder. Again, as the boat rocks, the rudder would move back and forth in the gudgeon until it thumps on either side. I wouldn't be alarmed by the noise in either case, but it could be an indication of wear that may become more troublesome later on. It could also be something in the bilge, or something slapping in the rigging, although rigging noises usually clang instead of thump. Of course, it could just be Blackbeard's Ghost, roaming the deck. Rick s/v DevOcean
