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It sounds like you have done a thorough job of troubleshooting and as Ozz suggests if you run the fan only it will eliminate the AC part, but not necessarily point to the fan. When I bought my new truck and installed a ham radio antenna and mount on it that had been on my old truck for years, I developed what I thought was a squeal coming from my dash, thought it might be the speedo or something else that was turning at high speed. It turns out the antenna and mount were creating a harmonic vibration at a frequency that sounded like a squeal. This was very bad as my dog would not get in the truck after having ridden in it once.
Changed the antenna to a different one and the noise was gone.
If you are convinced it is not a bearing, and you have checked both fans, then possibly a harmonic may be the cause, how or where I have no idea, I am just giving you something more to consider. Good luck on finding the problem
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