Thank you for pointing that out! I never would have really noticed that but I can absolutely hear it now. My guess as to why that happened is because I was using vocaloid 6 for the first time so I wasn't familiar with all the settings it had for EQ and stuff.
did you just know by listening that it was a 10kHz hiss because that is majorly impressive!
Also, What do you mean by "but what's this UFO"? I mean I know it stands for unidentified flying object but what does it mean in this context? I hope I'm not just missing something obvious.