The phrase "discordant note" means a sound or statement that does not fit well with the surrounding ones, creating a sense of disagreement or confusion. Full definition
The only discordant note in the photo arises from the contrast between your attire and the lush look of green behind you. (amidprivilege.com)
But that is absolutely the only discordant note in the predictably lush interior, where the attention to finish detail borders on obsessive. (automobilemag.com)
So why did it strike a slightly discordant note? (theguardian.com)