For the actual physical activity of knocking on a door, you can use qIp "hit (with hand, fist...)." But to include the notion of why you're banging on the door, add in HIl "make someone's presence known." The object of this verb is the person whose presence is being announced (so for announcing oneself, you'd use HIl'egh). This can be used when knocking at a door, ringing a doorbell, making an announcement and/or playing a fanfare on a trumpet like in a royal court, etc.

(QelIS boqHarmey – Alice im Wunderland auf Klingonisch, p. 232)