"Though Orientals are very jealous of their privacy, they never knock when about to enter your room, but walk in without warning or ceremony. It is nearly impossible to teach an Arab servant to knock at your door. They give warning at the outer gate either by calling or knocking. To stand and call is a very common and respectful mode. Thus Moses commanded the holder of a pledge to stand without and call to the owner to come forth (Deuteronomy 24:10). This was to avoid the violent intrusion of cruel creditors. Peter stood knocking at the outer door (Acts 12:13, 16), and the three men sent to Joppa by Cornelius made inquiry and `stood before the gate' (10:17, 18). The idea is that the guard over your privacy is to be placed at the entrance."
Knocking is used as a sign of importunity (Matthew 7:7, 8; Luke 13:25), and of the coming of Christ (Luke 12:36; Revelation 3:20).