Hmm it could be meddling from power wires too near to audio wires. Or they can be tangled under your desk or something.

Attempt to divide the maximum you can audio from power wires and confirm audio wires are not knotted. They can form a solenoid, this way capturing electromagnetic waves (radio, TV, cell phone) creating that noise on your speaker.

Also, attempt to verify if your audio connectors are not oxidated... both from your sound card and speakers/headphones.