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I have that problem on occasion... it's not consitant though. When I had that problem in my Corolla and my Sentra it lasted for about two weeks... then the tailpipe fell off... lol...
My guess is the piping hitting the heat shield. Might also try getting new exhuast hangers. Usually when they're really old, they get soft and let the exhaust shake more than normal.
it could be that. or maybe it was like mine where the muffler had rusted on the inside or something and pieces of metal were just loose inside , it was essentailly like a big rattle.