For my FC660C I've placed a few pieces Sorbothane in the bottom part of the case, mixed with some silverstone silencing foam also. They sorbo should be working well to dampen some of the vibrations running through the plastic of the case, and the foam should will be deadening any reverberations happening within the airspace under the PCB. I've also placed some sorbothane in between the plate and the PCB, stuck to the plate wherever there was enough space, in particular under the spacebar section. That seems to really prevent much of any plate ping.
I used 3204 for the sliders and the stabilisers. you only need a very light application though. All the sliders have Silence X Rings which does a bit to reduce the rattly sound of the board, there are probably more effective silencing rings, such as the Keyclack V2 rings, but they are thicker and will start to affect feel and tactility. The Silence X Rings are so thin that they don't seem to affect the feel very much, but do have a quite noticeable effect on the sound. They can be a little bit fiddly to get installed and to fit properly, but overall I would recommend them.
Definitely want to give properly silenced Topre a try sometime though..
