Sorry I have not responded to your question sooner. I've had some poor health related issues, which have kept me rather confined. Things are going better now, fortunately, and I have returned to sorting my truck out. In answer to your question, I have not found the reason for the tapping noise. I believe it could be a lifter or rocker arm, but am not able to determine this so far. I will keep investigating this and, should I happen upon the cause, will post my findings.