plank vs plump down

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

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

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

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

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

plank (noun)	plump down (noun)
an endorsed policy in the platform of a political party
a stout length of sawn timber; made in a wide variety of sizes and used for many purposes

plank (verb)	plump down (verb)
cook and serve on a plank	set (something or oneself) down with or as if with a noise
cover with planks	drop heavily
set (something or oneself) down with or as if with a noise