I may have found the solution to my headphones making this high pitch noise.
I've undergone quite a few replacements in my hunt to track down the problem component. I have replaced the internal feedback microphone with an OEM microphone. In the process, I accidentally broke the headband and replaced that. However, the feedback persisted.
In the teardown process, I accidentally damaged the right ear diver and replaced it. During this repair, I also replaced the internal feedback microphone with these replacements https://www.amazon.com/dp/B00LZV1ERG. Unfortunately, they were quite difficult to solder to the wires, and when successfully connect produced a VERY strong noise canceling effect that was very uneven from the undamaged left mic/driver.
The last microphone I tried in place of the original microphone was a gutted external feedback microphone.
With a crafting knife, I scored and removed the rubber. Do pay special attention to the positive and negative terminals, since they are not marked on the microphone itself. I soldered this microphone in place of the original, taking care not to expose the replacement to too much heat. I then reset the XM4s to factory settings and recalibrated the noise canceling.
Since replacing my internal microphone with this one I have not experienced the intense feedback. This is with typical use over the course of two weeks. The feedback would be exacerbated when the headphones were left out in cold humid rooms, when left charging overnight, or when the earpiece was left "half-off" my ear. In all circumstances that would typically produce feedback, I am longer getting any!!
Do note that this is technically a modification of two variables - a replacement of the driver and a replacement of the microphone. Though, hopefully others can provide their feedback and experiences.
Update (06/14/2023): Replacing the internal microphone with the gutted external feedback microphone has stilled worked!
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