That "straight line" above 15 kHz is very common problem. I will simply quote 2Bdecided's post from 2010.
CRT TV in the studio. That's the line whistle - the sound coming from the back of the CRT TV tube at the line frequency of the video signal. 15.625kHz for PAL; 15.734kHz for NTSC.
It's quite common in recordings. Occasionally it can be horribly loud, and people with good high frequency hearing can be annoyed by it. It doesn't "look" that loud in this case though.
In most cases, you can simply ignore it, it will be inaudible.
There are some exceptions, of course.
One of the worst ones I have encountered is "Justify My Love" from Madonna.
I am pretty sure every single person here knows that song. That song is very quiet, lots of whispering and this problem is very audible.
You can "fix" it by using notch filter or playing with eq.