The only way I know to make a boiling kettle silent is to use a room temperature kettle. This can be achieved by putting a container of water in an airtight box and then sucking out all the air from the box. Without any air pressure the water will begin to boil without any heat being applied. Also, since their is no air present (a vacuum) then sound cannot travel to the observer. This is then a silent boiling kettle. The only problem is that the water would be no good for making tea or coffee as it is not actually hot!