Why does touching one of my rca jacks to the side of the other one that's plugged in eliminate the buzz?

If one or both are plugged in from my computer to my stereo using a 1/8" jack to stereo rca cord, it produces a loud hum/buzz. Unplugging one and touching it to the side of the rca jack eliminates the buzz. I fixed it in place with aluminum foil, but of course its producing mono sound. Why does this get rid of the buzz? Is there some way to fix this so I can get stereo sound?