Did you build up a 5.0L pushrod stroker or did you stroke out your 4.6 to 5.0L? In either case, check your header/exhaust manifolds on the side of the engine that the noise comes from. If it's a pushrod 5.0L motor you may want to check out the lifters, if it's a 4.6 I agree with the person above who said to check the valve seats(seats or seals?). Another option, which I would HIGHLY RECCOMEND is a seafoam smoke show. Put some in your gas, some in the crank case, and vacuum line some into your plenum/manifold.