an axe to grind
1. A complaint or dispute that one feels compelled to discuss. I think the boss has a bit of an axe to grind with you over the way the account was handled.
2. A personal motivation or selfish reason for saying or doing something. It was boy's-club attitudes like yours that made my time at school a living hell, so yeah, I have a bit of an axe to grind. I don't have an axe to grind here—I just want to know the truth.
axe to grind
A selfish or ulterior aim: He claimed to be disinterested, but I knew he had an axe to grind.