Hissing/Rattlesnake like sound on speakers using Yamaha RXV 620.What could be the cause? I've isolated the problem by connecting just the speakers to the main.No other components connected. Not even the antenna for the tuner. When I turn up the volume the rattling/hissing increases. Its more of a rattle snake sound actually and not a hiss. I've used two brands of speakers with same results. Also plugged AVR in different sockets. Please help.Yamaha RXV620.