"Noiseless" can refer to a brand of typewriter (the Noiseless), or a series of typewriters derived from those designs produced by Remington and marketed by Underwood. These typewriters use a system of weights designed to make them exceptionally quiet.
The Noiseless was invented by Washington Parker Kidder in 19XX to ensure even and quiet printing despite the typing force. In essence, the user doesn't directly power the typewriter key all the way to the platen: instead, a small weight, following the momentum of the user's typing, would bring the slug through the last few milimetres to the platen. This gave the Noiseless mechanism the ability to accurately control the print quality and significantly reduced the noise that these typewriters produced.
The name "Noiseless" appears on a variety of machines -- usually Remingtons -- built from the 1930s through the 1950s. Some machines will also bear the Underwood name. Less commonly, some Noiseless typewriters were built by the Noiseless Typewriter Co. from the 1920s.
Somewhat ironically, the reduced noise was so effective that many secretaries, used to the loud noises of conventional typewriters, felt as if they weren't working adequately on a Noiseless. Furthermore, because the slug was stopped short of the platen, the typewriter was prone to skipping if the user had poor technique and feels as if it bottoms out early.
One can see the Noiseless mechanism in action here.