niggle

Concise Oxford English Dictionary © 2008 Oxford University Press:
niggle/ˈnɪɡl/

▶verb

• 1 cause slight but persistent annoyance, discomfort, or anxiety.

• 2 criticize in a petty way.

▶noun a trivial criticism, worry, or annoyance.

– derivatives
niggling adjective,
nigglingly adverb,
niggly adjective.

– origin C17: appar. of Scand. origin.