Far more to it than this. What case have you got, what hardware is in it, and in particular what cooling?
Noise reduction works best by working out the loudest part then silencing it as much as possible, then repeating.
Neither acoustic matting nor rubber pins will help if your graphics card is screaming. Rubber mounting of fans is good, and I do it as a matter of course in every build, but it isn't significant unless fan vibration is the problem. Acoustic matting is one of the last steps, once nearly everything else has been dealt with.
If you can offer details of the system, Id be happy to help.