plop vs plunk down

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

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

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

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

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

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

plop (verb)	plunk 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 something with a plopping sound

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