Hmm... probably some internal data signal bleeding into audio. I 
suppose the coming and going of the sound corresponds to the display 
changing on the Beomaster?
 I can actually hear some bleeps and buzzes on my BM6500 if I select an unused source and crank up the volume near to max.
Now
 what to do about it, I dunno... is it on radio as well or just the 
Beogram? In the latter case, I guess it could be a bad ground in the 
connecting cable.