niggle about (someone or something)
To make petty, trivial, or minor complaints about someone or something. The couple at table 22 has been niggling about their meal since the moment they sat down. Tom's always niggling about his boss, but I know he'll never stand up to her.
niggle about something
to make constant petty complaints about something. Please don't niggle about little things like this. This is just not important. Let's not niggle about it.