Tapping description wouldn't be octane related.
With engine idling, use a rubber hose on your ear and then place it around where you think noise is loudest, yes like a stethoscope.
See if you can localize it.
Yes, valve train or exhaust manifold leak are most likely sources
Fuel Injectors noise would most likely be described as a ticking noise.
If you can localize it then shut off engine, unhook one spark plug wire on closest cylinder.
Start engine, yes it will miss, but listen for tapping, if it is still there shut off engine hook spark plug wire back up and unhook next closest.
If tapping does go away with one spark plug unhooked then it is either an exhaust manifold leak or a Connecting Rod bearing starting to fail.
If tapping stays no matter what it is valve train issue