How to I fix the white noise coming from my subwoofer?

There is a constant humming coming from my SW, and i cant pinpoint what it is. The hum goes away when i disconnect the cable at the SW end and leave it in the AMP, but it stays when i keep it plugged it into the SW and unplug it from the amp. i tried tweaking volumes on my amp to no avail. I tried Googleing it but couldn't make heads or tails of what i found.