sound coming from subwoofer, even turned off

Have anyone seem anything like this? I have recently bought a NR-616 receiver and it seems to be correctly installed and working fine. However, it looks like the there is some kind of sound coming from the subwoofer, when the receiver is on (on cable, dvd, game, radio, whatever), and even when the receiver is powered off. I couldn't say what kind of sound it is because it's not that loud but loud enough to annoy anyone watching TV, or when the room is quite and the receiver is off. I thought it was some radio interference but I couldn't find anything in the manual or online. Any tip ? Thanks.

@dberni, is it a wireless subwoofer by any chance? If it is, it is interference generated by the subwoofer's wifi broadcaster. The sound I get from my wireless sub is a 'thump/thud' noise. As if something has dropped 'upstairs'. Changing the wifi channnel anbd moving the wifi broadcaster away from EM devices will help. Mine is a Pol Audio PSWi225.