niggling vs petty

niggling is not a noun while petty is a noun.

Word	Noun	Adjective	Verb	Adverb
niggling	No	Yes	No	No
petty	Yes	Yes	No	No

As adjectives, niggling and petty are synonyms defined as:

Other synonyms of niggling include fiddling, footling, lilliputian, little, picayune, piddling, piffling, trivial.

niggling (noun)	petty (noun)
	larceny of property having a value less than some amount (the amount varies by locale)

niggling (adjective)	petty (adjective)
(informal) small and of little importance	inferior in rank or status
	contemptibly narrow in outlook
	(informal) small and of little importance