You will get a hum on poorly manufactured stereos and/or speakers, its due to the connections within the stereo and/or speakers.
The connections will be interfering with other connections and cabling within the product so thats why you get a slight hum.
For a stereo this expensive its suprising that it does hum, as you would expect more for paying out just over £200, see
SHARP HTDV40H, the cheapest if over £200!
But to be perfectly honest, i wouldnt ever buy anything that was Sharp unless it was very good bargain. My favourite brands are Pioneer, LG & Sony.
Never had any problems with these brands,
Hope this helps.
Cheers.