I had a "tapping" sound on one ot two of my rigs. It was the rotor inside the cap that was making the noise. It chewed up the cap contacts on the inside. The rotor was trashed as well. Had to replace them both and no more tapping noise. One thing also comes to mind, you may have a defective distributor. Either the advance plate is not lined up or properly seated or if it has mechanical advance the springs may not be in correctly or one may be missing. I would put in the old distributor and see if it stops. If so, then its the distributor. I would return it and get another one. Just a few suggestions.
