I personally use felt feet/discs. They are typically used at the bottom of heavy objects to protect table surfaces from scratches. The added bonus they have over conventional rubber pads/feet is that the felt fibers get driven/screwed into the nut/bolt and serve to lock the nut/bolt in place and furthur isolate noise by insulating the screw and nut also.

P.S. they are cheap too. $1.50 for a sheet of 40 self adhesive discs

-=S_Klass=- I tweak... therefore I am.