I have recently purchased a HTS3201/12 2.1 sound hub. There is an issue with the unit. There is a constant hiss coming from the DVD / sound hub player unit. The hiss is still there even when all other appliances have been unplugged from the wall, cables disconnected and speakers disconnected. The hiss is audible in all modes when the unit is switched on, but goes away for a moment when switching between inputs. The hiss is not loud but is noticeable at low and medium volumes, but is drowned out at high volume. Please advise what can be done to fix this issue?