A visit to the public library has yielded a large amount of data.
“With Hidden Noise consists quite simply of a ball of string clamped between two brass plates held in position by four long screws, which project at the bottom to form four legs. The significant thing is that Duchamp asked his friend Walter Arensberg to place inside the ball of string a small object whose identity was to be kept secret; the only indication being the noise it makes when the Readymade is shaken.”
A Bruit Secret (With Hidden Noise): Duchamp assembled three in 1916; an admirer created a replica in 1963, and in 1964 Duchamp authorized a gallery in Milan to make eight more (to those, his wife, Teeny, added the hidden noise). Of the three originals, two were lost; the third one became part of The Louise and Walter Arensberg Collection and is on permanent display in the Philadelphia Museum of Art. The surviving With Hidden Noise was also the only one truly completed because no one had added a secret object to the other two before they were lost. Duchamp claimed that he never knew what Arensberg had put inside, whether a diamond or a small coin, and now that With Hidden Noise had become a sacred object in a museum, the noise was likely to remain a secret forever.
One cannot stay hidden forever.