You don't mention an audio system, if you had one, its sounds like "crosstalk" - which is when you hear an audio source at a low level even when it isn't selected. Do you have an audio system - if so, unplug it & see if the sound stops.
Potentially you could also get crosstalk from any source in a system - so if you have a dvbT tuner on your beovision that you dont use ( as you have sky) select it and see if it is on that channel. Bit of a longshot, but possible.
However, assuming there isn't an audio system, or a "dormant" tuner set to radio - it sounds like a shielding issue on the cables and your system is picking up RF interference - unusual to find it in a system like yours though. That would require searching for a problem withthe sheidling/maybe improving the earthing or at worst re-cabling.
Bit of a process of elimination required - firstly if you have anything set to absolute radio ( even if you think its off/don't use it) , then maybe a full or partial re-cable.