I moved your post to the general discussion forum in hopes of driving more responses to it. Don’t know if you dove in and ensured the rudder knock was happening at the bottom but there is also a bushing at the top that can come loose and cause vibrations.
See the bottom of this page: Rudder Stuffing Box and Bushing
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Bryan Bywalec
S/V Pelican, Hull 209
Siren Engineering, LLC