There are three ways to make a crossbow quitter.
The first one is to lubricate or add wax to the bowstring. This will prevent friction and will produce very little to no noise.
The second and best method is to use the silencing limb. A good quality silencing limb can go a long way to make your crossbow quitter.
The third method to prevent your crossbow from being noisy is to add stops at the end of the bowstring though not all crossbows come with their own stops, those that do have a great feature in this regard.