plunk down vs plank

plunk down is not a noun while plank is a noun.

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

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

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

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

plunk down (noun)	plank (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

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