plump down vs plop

plump down is not a noun while plop is a noun.

Word	Noun	Adjective	Verb	Adverb
plump down	No	No	Yes	No
plop	Yes	No	Yes	Yes

As verbs, plump down and plop are synonyms defined as:

plump down and plop: set (something or oneself) down with or as if with a noise

Other synonyms of plump down include flump, plank, plonk, plump, plunk, plunk down.

plump down (noun)	plop (noun)
	the noise of a rounded object dropping into a liquid without a splash

plump down (verb)	plop (verb)
set (something or oneself) down with or as if with a noise	set (something or oneself) down with or as if with a noise
drop heavily	drop with the sound of something falling into water
	drop something with a plopping sound

plump down (adverb)	plop (adverb)
	with a short hollow thud