Branch Die-off and Thinner Leaves on Sweet Gum Tree?

I have two sweet gum trees in my yard about 75 feet apart. This past year, the leaves have not been as dense, and one of the trees had 4-5 large branches die off. I had them removed, and some of the inner wood was blackened. No insect tunnels were noted. This tree is in a rather wet part of the yard except in July-August when we have very hot dry weather. The other is well drained with no limb loss. One non-arborist tree guy suggested carpenter ants, but there is no wood damage or saw dust. What is happening to my trees and what can I do about it? Thanks!

By Brenda L.