Last year in Moab a friend of mine went through and had the exact same sounds and thoughts you have, we were thinking it was something in the tcase until we removed the rear ds and the sound moved from the tcase area to a low growl from the 3rd. Pull the rear ds and put it in double low, as you are crawling along have someone crawl along next to the rear 3rd and see if it is from there, you may be surprised. That rear ds amplifies the sound and will make you think it is coming from the tcase.
