I was getting background hiss from time to time and instinctively knew there was a problem. Where was the hiss coming from?
I checked obvious inputs, and nothing was connected, so where was the hissing source?
I just knew it had to be on the input side, cos it was not there a few days ago.
I fired up Alsa Volume Control Manager and systematically worked through the inputs. I killed each in sequence.
Microphone and Mic Boost Input was set at 100%.
I muted both and silence prevailed.