Hey Scotty, I have a 2006 GMC Sierra 2500HD 4x4 crew cab 6.0L vortec motor with 220,000 miles on it.  Periodically when Im driving and the truck is warmed up, I start getting a tapping noise that sounds like its on the drivers side of the motor.  It lasts for a little while when Im driving and then just stops.  It doesn't do it when I first crank the truck and its cold because originally I thought it may be piston slap.  I can't find any real good advice on what this may be.  No codes are being thrown on the truck at all so no sense in scanning with my little OBD scanner.  I was considering having the engine rebuilt but dont want to if I dont have to.  

Also, my fuel mileage has dropped to 10.73 mpg average.  I used to get just over 12 mpg.  I have 285 size tires when this truck is equipped with 245 originally.  Not sure this is a symptom of the tapping issue or not though.  Thanks in advance!