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			It sounds to me like a typical top end noise.  My guess, and that's all any of us can give you without digging in deeper, is that it's a lifter/valve noise. 
 
 
Edit:  I just read that it went away.  If, indeed, it was the Marvel Mystery Oil, then most likely your original problem was a stuck/collapsed lifter.  (Although I have no idea why it would have advanced into a rumble)  
 
Great news that the noise is gone, though!!  :dance:
		 
		
		
		
		
		
		
			
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1980 Ford C-8000 Custom Cab Emergency-One Fire Truck 
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						Last edited by piper6909; 07-26-2020 at 06:27 PM.
					
					
				
			
		
		
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