Tap, knock, rap, thump, thud can mean, as verbs, to strike or hit audibly or, as nouns, the sound or effect produced by such striking or hitting.
Tap implies a light blow usually repeated (as to attract attention to one’s presence, needs, or wishes).
Knock implies a more forcible blow than tap , often a pounding or hammering (as on a door to call attention to one’s need for admittance or on a person’s body or head in an attack or a collision). In extended use knock may imply a sound or an effect that suggests a knocking.
Rap suggests a smart, vigorous striking (as with the knuckles) on a hard surface so as to produce a sharp, quick sound or succession of sounds. In extended use rap implies utterance that comes as quick and as sharp as rapping.
Thump implies the dull, heavy, yet resonant sound made by something that pounds or beats (as the fist upon a table or heavy feet upon the ground) or by something that pulsates forcibly and noisily.
Thud places more emphasis upon the sound made than upon the action which produces it and carries a less strong suggestion of repetition; also, it more often suggests the sound made by something falling and striking than by something being struck; otherwise it differs only by suggesting a flatter or hollower and less resonant sound than thump .