Hi jhunt17,
Welcome. No, your sub should not have an audible hum. You have what's called a "ground loop", which is a byproduct of the multiple power supply cables and a ground voltage differential (often the TV or satellite cable feed to your system is the culprit).
Anyway, read this article:(http://www.axiomaudio.com/tips_eliminate_hum.html) on some steps to get rid of it and here are some other things you can try.
Try plugging the sub into a different AC outlet in the room, one that isn't supplying power to your components (A/V receiver, TV, cable box, etc.). That might fix it.
If not, disconnect your cable TV or satellite feed at the point it enters the cable box. If that eliminates the hum, you can install one of these in-line ground guards between the cable TV feed and your box or TV.
http://www.tributariescable.com/catalog_view.php?id=172
Try reversing the AC plug (if you can) for your A/V receiver. Each time you make a change, try putting the ground screw back into the sub to see if there's any difference in the hum level.
Finally, if none of these approaches work, then this inexpensive Xantech Ground Breaker should solve it.
http://basshome.com/product_3965_detailed.htm
Regards,
