This might sound silly but electrical components are funky like that.
Try the .cap, make sure you can hear the high frequency, then unplug (not turn down, UNPLUG) the speakers/amplifier and turn off your monitor.
If the sound persists..... worry, I guess.
Sometimes badly isolated cables can generate sound on speakers or even CRT monitors (dunno the others), and that's harmless. Annoying but harmless.
On my old CRT I could HEAR MY MOUSE CURSOR going over bright lines. It was freaky.