First check the simple things; an exhaust shield or some other component could be loose, or you could have a stone wedged up by a shield or something. It's also possible though unlikely a baffle in your muffler may have rusted through and is loose. If a damaged muffler is the case, the sound will be come from the tailpipe and nowhere else. Next, you could also have an issue with a loose bolt on something somewhere, but good luck finding it. Check those basic things first though.