I've litened to it again and I've noticed it now. It sounds like there are two voices placed apart and it's not a voice moving around. I guess my ears are not used to hearing this effect. I would not have made this listening error if the voices placed apart were differents (two different singers). You know, if you listen to a person singing it makes sense to hear the voice come from one direction. If there are two singers it makes sense to hear the voices from different directions. But if you hear the voice of a single singer coming from two different directions it does not make sense to the brain. Because the brain thinks it is impossible for the same voice to come from two directions, it tries to fix the voice in a single place but it is not possible because it comes from two directions. You then hear the two voices as a single voice moving left right fast because the brain thinks that it makes more sense.
I think it may be something like that.
