hahahaha
not sure how i failed to notice that the RF receiver for my wireless mouse was in the port next to the audio jack.... that seems to be the culprit - have moved it to other side of laptop and buzz has gone.
my laptop (a zoostorm in clevo case) produces an annoying buzz when no sound is being outputted. if any intentional sound is produced the buzz stops and only resumes about 30 seconds after the song or whatever finishes.
i'm stumped!