Thanks in advance for reading:

I have a new ep350 sub, and seem to have an ever present, rather soft, hum. The hum is present when my receiver is off, with the cable removed from the receiver, and even after the sub has gone in to "sleep" mode. With the sub in sleep mode, I can feel a slight vibration on the heat sink of the sub.

Any ideas where to start with this?

Thanks for your help!