My BT telephone system has recently been plagued with a bad hum (loud enough to make communicating difficult). I've traced it to one of my STB's, as the hum goes when I unplug it. Is there anything I can try before sending the box off for repair, please? (It's a Panasonic DSB40 and, being part of a multi-room set up, has to be connected to the telephone.) Many thanks.