It's definitely exhaust drone (I can replicate it when revving the engine under no load). However, I've noticed it is worse when colder out, I suppose the internal muffler baffles may be less flexible and/or the body sound dampening may not work at low temperature as the steel is stiffer and resonates more.
I also took out the cargo cover a couple months ago which may have made it louder. I know it's louder when the seats are folded down. Maybe I could dynamat the trunk next summer.