Have you seriously considered the possibility of it being a mental issue, or have you just immediately dismissed that possibility every time it's brought up/any time that thought crosses your mind? Not a rhetorical question, genuinely asking.
Just from what you've said here, it sounds, to me, like you haven't seriously considered that as a possibility and have assumed it isn't. I don't know if you have any history with mental issues in the past, but if you don't, I know it can be kind of a hard thing to wrap your mind around if up to this point in your life, mentally you've been completely fine. But, issues can crop up at any point in life, especially if you've been through particularly hard times lately (don't know if you have or haven't, just saying that as a general statement.)
Put it this way, if someone is genuinely stalking you, that is a serious issue that needs to be addressed. If it's a psych problem, that's still a serious issue that needs to be addressed. Either way, this is a very serious situation. So, if it being a mental issue is not a possibility you've honestly considered, you should honestly consider that possibility, because either way this is a problem. If nothing else, even if you're absolutely certain it's not a mental issue, you should still see someone who's qualified to tell you whether or not it could be a mental issue just so you can absolutely, 100% rule it out as a possibility. No matter how certain you are, if you haven't seen any kind of a doctor about it, the possibility of it being a mental issue will always be there, and (as has been demonstrated here) people will always bring that up as the more likely option than someone committing their entire life to aggravating you with high-pitched noises.
If nothing else, if you were to talk to someone qualified, and they were able to rule it out as a mental issue, then that proves that some asshole really is doing this to you, which will only make it easier to resolve the situation. And then, conversely, if it does turn out to likely be a mental issue, well then there ya' go, there's your explanation. Win-win. So, I would suggest you should investigate the possibility of it being a mental issue, if that's not a possibility you've seriously investigated, since if it is a mental issue, then like I said, there's your explanation. And if it's not a mental issue and this person really is stalking you, then it would be a lot more difficult for people to deny it if you've talked to qualified medical professionals who have been able to rule that out as a possibility.
My mother had a period of time in which she was certain she was being stalked online, absolutely certain it was someone stalking her, and it turned out to be mental issues which have since been more or less resolved, so I have personal experience with that type of thing happening and know that it can sometimes be a result of mental problems, especially if said person is going through some really hard times. Speaking personally, I myself have also had periods in my life in which I've had auditory hallucinations as a result of mental issues as well.
This advice all applies whether or not someone is actually stalking you. I'd give this same advice whether I believed it was someone actually stalking you, or if I believed it to be a mental issue. That having been said, I do personally lean more towards this being a mental issue, really the biggest thing you've said that leads me to that conclusion is the fact that you've tried to record these high-pitched sounds, but they aren't heard on the recording. Unless the sounds are really, really, REALLY quiet (which doesn't seem to be the case, the way you've described it) or unless you're trying to record it with something that has a truly god-awful shitty microphone, you would absolutely, 100% be able to hear these sounds when you play the audio file back, even if you had to turn the volume way up to hear it. That's quite strong evidence towards it being a potential mental issue.
I don't say that to be dismissive whatsoever; again, if it is a psych issue, that's still a serious situation that needs to be addressed. I'm only saying that you should sincerely try to rule out the possibility of it being a psych issue, if that's not something you've done already. Doing so will only either give you a logical explanation for these happenings, or it will only make the case of someone actually stalking you stronger and more believable. And either one of those scenarios would be good for you in this situation, no matter what the true explanation is.