I can tell looking at your rest that it is one of the older versions, i had a similar experience with my first ripcord. Only I had mine setup up to drop down on the shelf. What your hearing is the rest coming to stop against the built in stopper and there is no way to muffle that sound.

Set up the rest and nock point the way it is supposed to be and then use moleskin or the new Simms dropaway pads or a combination. When you set it up properly the rest will hit the shelf before it hits the internal stopper, and you will experience less noise.

Dropaways are going to have more noise than the WB, but it is a tradeoff.