There is not a lot separating 15kHz from 17kHz with regards to human hearing, as pitch is perceived logarithmically in relation to frequency. If put into reference with music scales, it would only span a couple of keys on a piano, provided the keys extended an additional 23 keys beyond C8. [ Try a tone generator using a tool like Audacity and see if the headphones behave similarly. There are simply too many factors to know for certain why any sound is present beyond 15kHz on YouTube videos. What you are hearing may actually be 17kHz, but from all of the information I have found, that sound is most likely not coming from the YouTube content that was playing.