update: I managed to get the device booting correctly with a newly installed OS, and everything looks fine. But the squealing tone remains.
did you try powering from a battery or a different source, like a battery bank or different computer?
Hi there,
BAD cap or crappy power supply. try what cG said see what you hear 
HTH
GL 
PJ
On battery it should go away completely , or it means there is a bad cap in it.
the noise is constant on battery or any of the various USB power adapters Iāve tried.
Hi there,
Time to open it up and give a visual inspection.
look for any swollen capacitors or perhaps some corrosion near the power port. Snap a picture and post it up, we can look to. Could also be a cold solder joint, has it done it from the beginning?
HTH
GL PJ
also try a different AC plug for the power supply. with no other devices , if it is a power strip.
In my case it was the buzzer that was causing the problem.
I fixed it by stuffing something into the buzzer hole.
LOL,
Hi there,
Thatās hardly a fix though cāmon manā¦ 
more like fixed the symptom not the problem , pretty funny though. 
GL PJ
the XIAO Expansion board will do that too when the 3 pin floatsā¦ usually on some type of comsā¦