Do you hear that static sound when you connect headphones to the rear jacks as well as the front panel jacks?
If so, can you disconnect the front panel audio connector and try again at the rear panel?
If the sound goes away when using the rear panel jacks then it could be the cable routing to the front panel is allowing the noise to be picked up. You might try re-routing it as far away from motherboard components as it allows, and don't bundle those wires with any other wires especially wires to the USB ports. Many cases use bad wiring for the audio jacks that lacks shielding or even twisting for noise elimination. In which case, you might just disconnect the front panel and use only rear panel jacks. Many people do just that.