Muffler Life?

[zzvyb6]

That's fatigue. I suspect you have a weak cylinder so you are getting a vibration harmonic that's 'Q'ed up the the natural frequency of the cantilevered muffler. You can try to shift the frequency to a lower value by putting a heavy weight/collar on the outlet. Also, set your idle rpm up until the thing stops shaking is 1 more option. This should be pretty obvious...