Ringing sound throughout apartment will cause a mental break

Who do you call? Ghostbusters...

On a serious note, when you say you turned off the mains, was that just all power to your apartment? If so it wouldn't stop something like a noisy transformer / aircon compressor or exhaust fan elsewhere resonating through the structure. I have also heard of issues with LED lights producing these sorts of sounds - especially as the lights get older or are connected to dimmers (a family member had precisely this issue and no-one could solve it - not even the electricians. The family member then called me around (probably because I am an amateur audio engineer) and sure enough the sound was "everywhere" - it was all the LED downlights in the building.

I would firstly try to keep a diary of these sounds. Are they 24x7 for example or do they only occur at certain times of day or night / weekdays / weekends, etc.

You can certainly call an acoustic consultant professional but I doubt they would exactly be cheap (just the measuring equipment they need to do their jobs correctly costs a fortune - we aren't talking about the $200 meters you can buy at Jaycar). But then again if it is annoying everyone in the complex it might be in the best interests of the body corporate to consider this, especially if you've already had electricians come through to no avail.

I remember having a similar issue at my previous house (though low frequency) and after my own diligent diary keeping and chasing it down via a long process of elimination, I finally narrowed it down to the exhaust fans at a hospital about 3 kilometres away. High frequency sounds that seem to be everywhere are going to be tricky though, because the higher the frequency, the more directional the sound becomes. So the fact it appears to be omnidirectional is a concerning contradiction. Enough for me to say that only bad LED lights or tinnitus could be most likely cause if not everyone is being annoyed by it, though I will take your word you've done the correct things to eliminate the latter possibility (i.e wearing noise isolating ear muffs late at night which would make tinnitus much worse or alternatively substantially muffle the offending "real" noise).

I tried to post a link to a random place I googled who were acoustic consultants but the censoring filter wouldn't let me do it. But you'd be looking up environmental acoustic consultants and finding one that can analyse noise within a residential apartment building.