sound a sour note

To be or come across as negative or pessimistic, especially in way that ruins or dampens the experience of others. A: "We just had our best quarterly sales figure in years!" B: "I don't mean to sound a sour note, but we'll need much better numbers than that to save the company." A: "Your steak was only OK?" B: "Yeah. I mean, I don't like sounding a sour note, but it cost nearly $50, so you expect it to be the best dang steak of your life!"