We viewed your photos.  We notice some lichens on the trunk (which are not a reason for any decline) http://extension.umd.edu/hgic/lichens-trees-and-shrubs, some old damage(old wound) to the trunk, and possible insects like carpenter ants. They like to nest in damp wood but they do not eat the wood like a termite. If the tree is putting out new shoot growth and looks healthy,  you can let it be.

However, we are not on site and you may want to contact several certified arborists for an onsite diagnosis regarding the health of the tree and the best way to proceed. http://www.treesaregood.org/
mh