unknown tapping under sail

My 26C has developed a tapping when under sail. It only is present when shift is in forward or neutral. No very loud but rymthemic while shaft is turning when moving forward. the boat has the standard hw50-2r transmission. The local boat shop can not locate any problems while testing at the dock. Any ideas?
thanks
Charles Webb
SV Sanctuary

NS26c 162

Sounds like the noise is coming from somewhere along the shaft or the prop. Most of the shaft and various other parts are outside of the boat. Do you have a diver? That is almost unheard of in Oregon where I came from, but down here they are a necessity. (The marina will not let us dive on our own boats even if I were so inclined.) I have a diver who cleans the hull monthly. He also replaces zincs as needed and inspects. If I have a question about something I can ask him to look at it specifically. If I’m not in a hurry he will do it on his next visit at no extra charge. In a case like yours where you might not want to wait, he would pop over sooner from another boat and check on it for a reasonable fee…