I have seen these many times on maples and it seems to be just an abnormal growth.  I have cut them open to find normal tissue.  I have never found a reference explaining why this occurs.  They are usually hard sound wood.  In a few years as more bark tissue develops and bark becomes thicker along the trunk of tree these may not be noticeable.  At this point I would do nothing since it is not harming the health of the tree.