By the way you describe it, it could be the same problem I had once on a Ford Ranger---I thought the same things you say. It turned out to be the heat shield/cage around the catalytic converter rattling and vibrating. It sounded like some horrible metal-to-metal contact or maybe something dragging on the pavement. I stopped the noise by installing two very large hose clamps tightly around the thing to stop the rattling and that fix worked for 150,000 miles. Maybe it will work for you too.