Hello, It very well could be internal, I don't know. I suggest a rigorous Seafoam cleaning to help eliminate some possibilities. Kinda like a controlled test. Our single port upper plenums have a vacuum hose port top center, just past the throttle body. Remove that hose and you will see a plastic port sticking out. Get a small piece of tubing and a funnel to add Seafoam in through there to soak upper intake plenum evenly. Do this while engine is warm not hot. Waite 15 min and repeat until no more smoke out of exhaust. When you start the car again do not depress throttle until it cranks over(Prevents cleaner from running back down air intake & MAF sensor). Do this in addition to sucking up cleaner through pcv valve and pouring can into gas tank. Seafoam runs about $7.80 here.
Trust me, this will improve engine performance enough to help diagnose. If you do the cleaning and are able to add 93 octane to eliminate the tapping sound, you have some info to pass along to a mechanic.
Hope this works out. Please update.
'97 3.8 Pacific Green
1996 SVT COBRA