How to get rid of the fan noise in the Xiaomi Air Purifier?

Xiaomi Air purifierXiaomi Air purifier is a wonderful device that not only passes room air through a filter that absorbs harmful substances, but also measures air pollution, temperature and humidity, and can initiate some smart home actions depending on the situation (even an alert to your smartphone when the air became smoky). It is a very good smart device, and it works almost silently.

However, after 2-3 years of using, especially at nighttime, you will start to hear a periodical fan noise. What is the reason, and how to restore silence?

Even if you set the Mi Home app to turn off the air purifier at night hours, you'll be surprised to find that you still hear this sound every 10-20 minutes, with the large fan remaining still. Apparently it's not this main fan, it still runs silently. There is a small removable cover on the back side of the air purifier that hides a unit that analyzes air quality:

That's where the little fan hides, periodically sucking in air for analysis! And it gets very noisy over time. What can be done? The easiest way to do is to insert a toothpick under the blade of this fan without disassembling anything. This may be a temporary solution of the situation, but a stopped fan under voltage is not very good thing. So it is much better to remove the plastic cover, remove the metal block of the analyzer, open it up, and disconnect the fan. Here it is after removal:

Even without this fan, the air purifier reacts well to changes in air quality. But you can buy a new fan (same or better quality), or order, for example on Aliexpress, a whole new air analyzer unit, it is not very expensive.