"Can anyone explain...."

No, not with certainty. At least not with the information you've provided thus far. And even with more info from you, diagnosing something like this successfully via the internet is a shot in the dark. We're all guessing.

Does the 'tapping' vary with engine speed? Does the sound change when the engine is cold started vs. running fully warmed up? Does the sound change when you're accelerating vs. decelerating? Does the sound change when the engine is under load vs. not under load?
"Change" could mean the volume of the noise, the tone/quality of the noise, etc. "Tapping" could describe the marbles-rattling-around-in-a-can sound of detonation. It could be the deeper, more ominous sound of rod knock due to a rod bearing having spun (or being worn out). It could be the lighter 'ticking' of the valve lash being too big. It could be the slightly more ominous sound of a cam lobe/lifter being worn beyond proper tolerances. Some of those are repairable pretty easily; others, more troublesome/terminal/expensive. My dad hit a piece of debris in the road once, and afterwards the engine was making a "tapping" noise. It varied with engine speed. Turned out that a small inspection plate on the bottom of the bell housing had been bent so that the flywheel/starter teeth were lightly hitting it when the engine was running. He loosed the one bolt that held the plate in, bent the plate back to "straight" eliminating the contact with the flywheel, and the noise stopped. No harm, no foul. 10 minutes of repair time. It could be something like that. My daughter's 760 wagon had a "tapping" noise. Turned out that the motor mounts had worn out allowing the engine to sag so that the fan was hitting the shroud, cracking it. New motor/tranny mounts and a minor repair to the plastic shroud. No more tapping. It could be something like that. Or, you could have a rod/crank bearing that has let go. Or valves that need adjusting.

Your best bet is -- a mechanic you trust. If your mechanic has heard the noise and thinks the valves need to be adjusted, I can only think you're asking us here because you don't trust the mechanic. My counsel is find a competent mechanic you trust and follow his/her lead. Good luck - hoping for an inexpensive fix.