plop vs plump down

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

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

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

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

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

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

plop (verb)	plump down (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 with the sound of something falling into water	drop heavily
drop something with a plopping sound

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