Loud Tapping

Can you determine if the tapping matches the engine RPM or is one-half engine RPM? Lifter and valvetrain chatter is at half the engine RPM and often the chatter goes away at higher RPMs. Noise coming from a rod is often most obvious at an even RPM, not accelerating or decelerating. A balance test can isolate a connecting rod noise by disabling a spark plug one at a time with engine running. when the offending cylinder is cut off, the noise will go away or be drastically reduced. (along with a connecting rod, a piston pin/wrist pin noise will also be reduced by a cylinder balance test, but pin noise tends to have a 'double' knock). Bottom end noise like piston slap and crank/main bearings are hard to miss, and occur equal to engine RPM. As you feel yours is valvetrain, I'd pull the covers off and check for clearances on valves and lifters, check springs for proper tension. You don't mention mileage on the engine, if it's excessive, may need an overhaul on heads.

Investigation is needed. Remove a valve cover and look for damage or excessive valve clearance.