Also check if you have plugged in a wireless receiver in back near the audio on mobo. I exp'd noise whether plugged in to front or back I/O USB and realized it was a micro USB receiver i barely noticed and even forgot about for my Logitech wireless keyboard and my wireless mouse. I go back n' forth between wired k & M when gaming and reg usage. Had an epiphany and looked at what had chg'd, realized noise appeared when wireless was used. Upon playing with Windows Media Center and a new remote for internet tv, radio, etc. I discovered all the noise-horrible, sounded like digital rain! No matter if I used my headphones or spkrs, it was there. Once i unplugged the wireless receiver, all noise disappeared!!!
I was thrilled not to have to delete audio Realtek HD drivers and update w/newest ones as some were doing, i also wondered if my ASUS 6890 was causing issues as the fan was going and then i wondered if my Icy Dock and its fan or location was an issue. My Motorola modem and Buffalo router were all in close location as well. I was going to unplug the Icy Dock next as that had been a purchase months ago and i wondered if that was an issue. Before having to do any of this with my last move to update Realtek drivers after I cleaned computer good w/drivercleaner. Very happy to say that the easiest thing resolved the issue and id begin there if anyone here goes wireless. Good luck!