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.
Hi,
I find my Creative MegaWorks 650 has a constant noise when turned on. The noise level is independent of the volume control on the speaker system and the volume control of the sound card. When I use my headphone with my Creative Audigy 2 NX(USB external), i can't hear any noise even at max volume. I think it's the connector between the Satelite spearks and the amplifier. Is there any trick to connect the speakers better?
Does anybody have some clues of what's going on?? I just bought my MegaWorks 650 brand new at a sale for $99.
Thanks a lot!!

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic