Sounds like induction, is your power strip near the woofer? Also check to see that the ground isn't touching something. The outside of an RCA connector is the ground so make sure there's nothing touching it. Also make sure all your connectors are pushed in all the way. Sometimes it's possible for an 1/8" connector to short if not in all the way. The fact that your headphones work rules out the sound card. Your problem is most likely just a bad connection or induction. If it's not either of those then it may be time to open up the woofer enclosure (if possible) and look for shorts (ie: solder bridges or touching wires) or faulty connections.