niggle
nig-gle
noun
A minor, persistent complaint, worry, or physical discomfort.
"He had a niggle in his knee after the run."
verb
To cause slight, persistent worry or annoyance to someone.
"A small doubt began to niggle at him."
Thesaurus
Synonyms: pettifog, bicker, quibble, brabble, squabble, fret, fuss