Tiny High Noise From the Right Fan

Hello,

Recently, since maybe a month or two (or six) ago, the right fan started producing the sound you can hear in the video I made when I removed the panel.

It's quite annoying. I blowed through the interior and partially cleaned up the white powderish dust inside. Until then, I'd ocassionally hear a crackling sound as if some object rubbed against a surface, or a small solid fragment got loose. I'm not sure the crackling won't reemerge.

With the back panel opened, I turned on the Mac and boosted the fan speed to the max with Temperature Gauge that I have running 24/7/365. The sound is normal at such speed. Once it's low, I hear that tiny sound. In fact, it's coming right now. When TG increases the fan speed, the sound modulations resemble those of the jet engine as the aircraft is taking off.

So, what is it?