It's hard, because the Model M relies on a steel spring buckling, which is inherently noisy. You can dampen some other sounds (like the keycap hitting the key's

It's hard, because the Model M relies on a steel spring buckling, which is inherently noisy. You can dampen some other sounds (like the keycap hitting the key's fixture, either with grease, or permanently with rubber rings), but with how little space there is in a key, it's hard to silence that particular sound. IBM themselves went with regular rubber domes for their "quiet touch" Model M variants, which are unpopular due to not being mechnical.