Yes, it's the XW alone in the tone mode. I've tried to set tone mode as default when you power on the keyboard. Then this effect appears immediately and I have to switch to the perform mode and back to make it sound properly. UPD. I've found out that it's the modulation effect and it appears when the modulation wheel isn't set to 0. So it solves my problem, I simply will set it to 0. Sorry, I'm a newbie in synthesizers:)