This may be a bit dated, but to announce oneself by one's fist upon a door in England was to 'knock up someone' whereas the same term in Yankee-land was quite different: to get a girl pregnant. A certain amount of titillation in England as I'm sure they knew the American take was taken advantage of on an old Goon Show:

"Hel-LO" I'm terribly sorry to knock you up so late! They all say that!