Singing crooked?

Sometimes I sing with the laptop in the background and then the song runs on the laptop. From time to time it happens that when I sing along it sounds so shrill or as if the sound vibrated or rather vibrated. But that's only when I don't sing with the song alone. Is it because I sing extremely wrong or where does it come from?

What you notice is probably the tension that arises when you sing just next to it. Without an example, however, it is not clear what kind of problem you have.