Standard rules for (Soviet) embassy staff during the cold war included writing secret documents with a pencil rather than a typewriter because the text might be deducible from the sound of the typing.

Of course the writing would also be done in a box suspended inside another room with white noise generators in the intermediate space, just in case the text might also be deducible from the sound of handwriting.

My source for that might be one of Viktor Suvorov's books.