Any way it could be the latches on the cowl?  My Johnny will cause the latches to buzz at a certain rpm.  The solid mounts on the Merc buzz the entire boat, LOL.   Someone on the board recently had a tilt pin that was rattling.  If you could have someone drive the boat while you are back at the engine checking on things, it might help.  Best of luck.
		 
		
		
		
		
		
		
			
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1996 -19' NV Flats  115 Mercury 4-stroke 
1983 -20' Wellcraft Center Console 250 XS
			  
		
		
		
		
		
	
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