Sound comes from the upstroke and the downstroke of the switches and echoes throughout the case.

If you or anyone reading this comment would like to build a near silent board I would suggest getting some Silent Alpaca switches or U4 Silent Tactile switches with switch films applied. Make sure to use all the sound dampening mods available (foam/tape mod/polyfill etc) to deaden any echo inside the case you are using. Avoiding cases that have a lot of echo will help also. Plug the back of the stabilisers with generous amounts of Krytox 205g0 and straighten the wires.

If you do the above the barely audible sound signature of the keyboard will resemble the soft pitter patter of rain.