[Dave in MN]

Machineman64,
This problem is frustrating isn't it! Believe me I know. Please let us know if you have completed any recent modifications to the engine just prior to the noise developing. I call your problem "cam walk"...the noise or tap is the thrust surfaces between the cam and the block slamming back together when the force pushing them apart subsides. This condition can also be caused by a worn center cam journal or worn block bore in the same area. Pay attention to James Rogers, Kelly's Restoration and Patrick L. as they make valid points as to possibilities. I would echo their comments/suggestions. I have been in your shoes...many times with the same problem when I work on others cars...if the cam plunger spring has been replaced and even shimmed and the problem still persists, the cause is probably one of the items these gents suggest. You can stop the noise with what you are looking to do but the best fix may be to find the real cause and correct it instead. Hope this helps. PM your phone number to me...I'll call and elaborate a few of my experiences and solutions.
Good Day!