OK I hear the cereal sound... Can there be something loose in the muffler? Sounds a bit like a rattling sound being created on the back pressure at first. another thought is what about the flapper in the exhaust manifold. If that spring is a bit weak could that be 'bouncing' a little and sending the sound on down the line?
maybe a stethoscope or long screwdriver at the heat box on the manifold would confirm or dismiss that one?
otherwise it beats me...

Give me ambiguity
or give me something else