The main culprits are 2 identical desk-type fans (frequently moved around) which aside from the blade noise make irritating high-pitch tones, and 2 (250mm) ceiling exhaust fans which make even worse high-pitch tones. Said tones have nothing to do with the bearings (the desk fans are sleeve bearing anyway; the ceiling exhausts are ball bearing, but the tone stops as soon as the power is turned off).
Working...
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