I have that exact same problem. I've narrowed it down to the tail pipe. And the only time it goes away is if I turn off the engine. I've been told that is what engines do, they make noise - but i'm not sold on the thought..
But on a serious note. Check clearances the whole way through. Have you put on any high clearance skids lately or anything else that would have moved the transfer case area up or down at all? Make sure nothing is hitting the heat shields.