zerozero - your last post gave a good clue - travelling on dirt roads at high speed. I have had stones wedge up between the muffler and heat shield on VW Jettas causing the same sort of sound - the muffler has to flex a little at moderate acceleration for the stone to vibrate - stronger or less acceleration jams the stone so it does not vibrate- the twist on the muffler has to be just right- physically removing the stone or having it vibrate out usually cures the problem. Hope this helps
Morris