That is a sign that a branch was removed too close to the trunk and the normal defenses of the tree, which are in the round, raised part where the branch is attached to the tree, we cut off.  The tree is trying to compensate.  Try sticking a screwdriver into the heartwood.  If it goes in easily, the heartwood is already rotting.  You need to have a certified arborist take a look and see if anything can be done.  Go to this web site and click on Find An Arborist.  Put in your zip code when requested and a list of certified arborists in your area will appear.  https://www.treesaregood.org/