Speaker/subwoofer hum?

I am wondering if and how I might be able to stop the low hum coming from my computer 5.1 speakers. The hum can only be heard when no music is playing and the room is silent, however this is a problem as I need to keep the speakers on overnight (sounds loud when trying to sleep).

The hum only comes from the subwoofer, so I don’t think it is related to the quality of the speakers (or at least not entirely).