If you can get to it with a stick, you may be able to blast it with compressed air to clean it. That MIGHT work, or the bearings or bushings may be shot.Used my Bushnell penlight to show an older co-worker where the horribly annoying, loud buzzing sound from the front desk comes from. A pair of stacked monitor boxes have a fan built into the back of the desk that sometimes acts up. (Some idiot constructed the fan around the desk over 20 years ago, if not longer. With no way to access the small fan for cleaning.)
. Awhile back, I discovered the real culprit. I also discovered that if you jam the wooden stick that the main work-keys are attached to into the blades of the little fan, the insanity driving noise; stops!
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Over the years, I had a few computer power supply fans start to make noise, and hitting them with a leaf blower cleaned them well enough to silence them. Although, I have replaced a couple computer tower fans.
Good luck.