Re: Possible lifter noise?
Usually lifter noise will happen at idle, and go away as you give a little gas. The lifters pump up as the oil pressure increases with RPM, and the noise goes away.
Does it change only on rpm, or does load affect it?
IMO you probably have a leak on one of the exhaust manifold/collector gaskets. Sometimes the "tapping" is just the pulse of escaping exhaust gases, and under certain velocities it leaks worse than others.
Look for an exhaust leak, and see if this is something that you can recreate in park or without load on the engine.
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'66 Short Step / SD Tuned / Big Cam LQ4 / Backhalfed /Built 4l80e / #REBUILDEVERYTHING 
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