I like what Fred wrote – good points, all.
I like to really isolate where the noisiest points are with action return noise.
Knuckles can be made quieter by needling through the core felt side to side right at the flattest point.
Wippen heel cloth can be made very quiet by flipping the stack over and putting a few drops of warm 50% alcohol/water in the capstan dimple.
One thing I disagree with is regarding metal rails making more noise – in Kawai actions the spruce keyframe is a big part of the action noise being radiated into the air, not the action rails. If you try tapping or thumping on action rails in various pianos, I find that the wood rails make just as much noise as metal ones - often more! The Kawai rail is especially rigid and non-vibrating (look closely at how it is made), even when old like this – thumping on them transmits noise to the keyframe, but doesn't make any noise within the rail that I can tell.
Finally, room acoustics REALLY contributes to how much action noise one hears. A piano with the tail in the corner of the room especially, and a live environment with wood floors etc., will really boost the amount of action noise heard. I have had some people just move the piano out of a corner and fixed the complaint that way.
Good luck with it!
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