niggling vs fiddling

niggling and fiddling both are adjectives.

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

As adjectives, niggling and fiddling are synonyms defined as:

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

niggling (adjective)	fiddling (adjective)
(informal) small and of little importance	(informal) small and of little importance