[Dirty Harry]

Do you have the large oval air cleaner? I had an instance, years ago, where a boot on a plug wire had failed. The air cleaner base was just above this boot, where the spark plug wire pushed into the distributor cap. The spark was jumping to ground, through the wounded boot to the air cleaner base, causing what sounded like a somewhat rhythmic tapping noise. You might try watching your engine running in the dark and see if you notice any spark jumping to ground. In the noisy engine bay, the snap of high voltage can sometimes sound like a mechanical tick. The stethoscope might not pick that up clearly.