I suspect you have a ground loop. It happens when one or more of your components have different grounds. Most often this is caused by the cable television feed. Cable companies use a different ground than your house and so induces the ground loop and hum that is often heard in subwoofers. I can't tell what your system components are but if you have cable TV the first thing I'd try is disconnecting the service cable and see if the hum goes away. If not, the process is to disconnect as many components as necessary to get rid of the hum and then start reconnecting them while listening for the hum. When the hum starts you have identified the offending component. If it is the cable there are ground breakers for coax cable. I had this very problem and solved it by breaking the ground on the actual TV cable. I hope you find the source of the hum quickly.