How young are you? (no need to answer..)
My guess is you are hearing the MD oscillator. If you're still young enough to have excellent hearing, you might be able to hear frequencies as high as 15 to 20 kilohertz.
As you get older (30+), it becomes much more difficult to hear high-pitched sounds. Even under 30 if you don't protect your hearing (guns, concerts, headphone volume too high, etc..)
Old analog TV sets used to have a horizontal line rate of about 15,750 Hz that you could hear "whine" on some models. Without even looking, you could tell if someone turned on the TV.
This would have been the 1970's / 80's and earlier.
Check your coil connections (wiggle them at both ends) to see if the amplitude changes (i.e., louder or softer).
If so, then verify you're making good contact. Otherwise, you could have a loose ground or shield somewhere in the electronics section.
It may also just be a design issue, and you just never noticed it before.
Either way, if it is the oscillator, I doubt it would really affect the Ace 250's operation.
One other "possible" explanation is that something EXTERNAL is interfering with your detector.
By this, I mean strong external radio-frequency fields (like from a nearby AM/FM Station, etc..).
This is particularly likely if you're hearing music or some other program audio, but less likely if it's just a steady, high-pitched whine.
Another possibility is the Ace 250's audio amplifier is oscillating.
You might be able to rule this in/out by seeing if the noise goes away at lower volume settings.
It's not conclusive, but many audio amps tend to oscillate 'only' when they are at or near maximum volume.
Though some designs never oscillate, and some are so bad they may as well be oscillators (not amplifiers).
BTW: An audio amp can both oscillate and amplify sound at the same time.
It's not quite analogous to "runaway" oscillation (squeal) like you might hear if you placed a microphone in front of a working speaker (amplifying the mic).
