Sky+ Hd interference sound problem - Please help

Hi all..

I have a sky+ hd set up
The RF1 goes to the main tv in the lounge via the amp - not a problem at all touch wood....
The RF2 goes to a booster which sends the leads out to the tv in the dining room, bedroom and the 2nd bedroom. On all these tvs I seem to get a very annoying vibrating interference sound when the gaps come on between breaks and on very quiet scenes on the tv programmes. It's only faint but seriously annoying. I have replaced the booster thinking this was the problem but it's still the same... Sometimes the noise goes completely and other times I can't bear watching the tv's apart from the one in the lounge...
I even unplugged all the electrical equipment apart from the sky+ hd to see if it was an electrical interference but was still the same.
Please can someone tell me what the problem could be and how to fix it???

Thanks