I have a MBP. It has a mini-plug stereo line-out/headphone jack. Headphones sound fine when plugged in. However, when I plug in a mini-plug to 1/4" Y cable in, and send that cable to my desktops external sound card (firebox), electrical noise becomes very prominent (eg. if I move the mouse, noise changes, etc). Is there a way to fix this?
sample of affected audio: