I don't think it can be a tool that has been dropped or lodged within the engine [compartment]. The tapping noise happens at idle (when fully warmed up), drive and as stated in my last post, in reverse - where it's the loudest.
My Dad had the timing belt and water pump replaced last week at the dealership. They thought it was the fan that was causing the noise and replaced it as well. When they finished and got the engine up to operating temperature guess what, the tapping was still there.
That's when they came up with the diagnosis of the Main Bearing.
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2004 Tundra DC SR5,
Blue Marlin, TRD Off-Road, Tow Package, All Weather Guard. The Rest is Stock and Damn it, I Love this truck!