Subwoofer not working, but smaller speakers are?

I got altec lansing speakers as a xmas gift. when i tried them at my parents house with my ipod, they worked great, but when i came back home and hooked them up with my laptop, they aren't working properly. the subwoofer just buzzes constantly as soon as it is powered, regardless of what it is hooked up to (ipod,laptop, etc). the two smaller speakers play the music regularly though. i've tried plugging everything in to different outlets but it still keeps buzzing rather than playing the music. help!?