knock vs knock against

knock is a noun but knock against is not a noun.

Word	Noun	Adjective	Verb	Adverb
knock	Yes	No	Yes	No
knock against	No	No	Yes	No

knock (noun)	knock against (noun)
the act of hitting vigorously
negative criticism
a vigorous blow
the sound of knocking (as on a door or in an engine or bearing)
a bad experience

knock (verb)	knock against (verb)
find fault with; express criticism of; point out real or perceived flaws	collide violently with an obstacle
knock against with force or violence
rap with the knuckles
deliver a sharp blow or push
make light, repeated taps on a surface
sound like a car engine that is firing too early