Computer subwoofer (Altec Lansing) won't stop hitting when it's plugged in to outlet. non-stop bass constantly?

I purchased an Altec Lansing (VS4621 Octane 7 Speaker system) set of computer speakers with a woofer, and yesterday i came home and i heard the bass thumping non-stop. i tried to turn the volume down and everything but it was just constant bass and i had to take the plug out to fix it. I tried it this morning but it still does the same thing. does anybody know why it is doing this?