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...
	
		
			
		
		
	
				
			