just throwin it out there but it could possibly be something inside the wheel/tire: for example i had a f250 come in at work and it kinda sounded like your above description. i found water and a rag inside. you may think this is way out there but truth be told its happened a few times in my shop. other than that i would start with easy stuff to shoot at it like getting your tires road force balanced. this indexs (sp) the high and low spots on the wheel with the high and low spots on the tire. anyways keep us/ me posted on your findings.