John, Have you tried playing the offending notes at various levels of force with a finger firmly on the string to damp it? Then play adjacent, non-offending notes the same way. This should isolate action sound (including keyframe), leaving strings and soundboard assembly out of the equation. If there is a significant difference in action sound between offending and non-offending notes, it should be fairly obvious. It should also be possible to hear with more acuity just what the difference is (slap, click, etc.), a help in diagnosis. If there isn't a difference with strings totally (almost) damped, then you start looking more at soundboard/plate/frame, etc. Glad it's you, not me. Regards, Fred Sturm University of New Mexico
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