I have installed my Soundbar 700 but it has a background hiss. I've turned off every appliance in the house one by one in case there is any interference and nothing gets rid of it. I've even taken it next door and tried it in a different house and it's still there. It starts as soon as the bar is switched on, you can hear it when there is quiet between speaking on TV programmes and if you turn music down really low. Anyone got any ideas or do I need to return it?