As someone who has lived most of his life south of a line drawn from The Severn to The Wash - the great linguistic and cultural divide in England - I was not familiar with the expression knocking on.
My grandson and his friends in Manchester will ask their mums if they can go and knock on for Jack, or report "Ahmed is knocking on for me". Or sometimes they simply just go knocking on for all and sundry. It means that they are going, sometimes from door to door to see if their friends are available to come and play.
So having identified it to Lancashire, where else do they knock on?