Recently, when I first turn on my Sony amp, I get a buzzing from the speakers. This can be turned on and off by using the speaker select switch on the front, and does not get louder or softer with the volume control. Also, it goes more quickly when playing something, as opposed to when there is silence. Does anybody have an idea what might be causing this? If so, do you know if it's a serious problem that will end up showing up as something much worse? Any help much appreciated! Mark.