the dealer told me that the 5.3 engine has makes that noise is this a common problem
my friends truck has developed a knocking type noise a piston slap on 2006 GMC Sierra 1500
tons of problems with lifter noises with the 5.3. GM has been replacing lifters and changing the viscosity of there oil to extend the life of the lifters.
I've never heard anything like that I would get an 2nd look at it. Bad lifters would give off a taping sound not a knocing sound .
These engines due top close tolerances are known to get carbon build-up which then causes a knock when then carbon on the piston hits the head. GM reports that this condition cause no reduction in engine life. If this is the cause of your noise you could get5 the motor decarbonized (or use seafoam your self).