hello, We traced it to the agrafe (they have that fixture for the 4th string)
My friend told me that possibly the inside of the agrafe holes have wear and the string may touch the end of the little barrel of the added hook (that is a second piece inserted (with force ?) in the side of the agrafe.
I heard something about those pianos but it related to the fixture on the bridge, for the aliquot wire.
There is a parasitic noise immediately at the attack, a metallic buzz - can be lowered by damping the front segment but still a little present. pushing laterally on the wire lower or suppress it.
There are a few notes exhibiting that noise (We will try to record that)
Highly annoying, the piano is rebuild with new action, new polyester, nice tone, but those 3 notes may be a sign of other trouble coming later.
Honestly I dont know how those agrafes are machined,my friend told me the added support is just inserted in a hole on the upper side of the agraffe (then some of it may be attainable if the inner holes have enough wear.
No change by pushing on the "hook" (for lack of a better term)
Nothing wrong with bridge/bridge pins, nor the fixture for the 4th string on the soundboard.
That problem may be specific and known by Bluether specialists.
I dont know if new agrafes can be find ... very possibly at the factory, the design may have changed.
I hope my friend will not need to change the whole agrafes in the section, but... very possible in the end.