flap down vs slam

flap down is not a noun while slam is a noun.

Word	Noun	Adjective	Verb	Adverb
flap down	No	No	Yes	No
slam	Yes	No	Yes	No

As verbs, flap down and slam are synonyms defined as:

flap down (noun)	slam (noun)
	an aggressive remark directed at a person like a missile and intended to have a telling effect
	winning all or all but one of the tricks in bridge
	a forceful impact that makes a loud noise
	the noise made by the forceful impact of two objects