rap on (something)

rap on (something)

To strike something with one or more quick, light blows, as when getting someone's attention; to knock or tap at something swiftly and lightly. I thought I heard someone rapping on my window in the middle of the night, so I was wide awake with fear until morning! The wind was causing a tree branch to rap on the door in a most unsettling manner.
See also: on, rap