Hi Tony.
Our old MKII had a similar trait, (low-frequency vibration) but it was only evident if the board was too deep for the point-of-sail (e.g. fully down on a reach). If the boat was properly trimmed, it didn’t do it.
Good luck with making a new CB, you can get the drawings from the association http://wayfarer.org.uk/index.php?option=com_content&task=view&id=40&Itemid=163
Dave