There are a couple of scenarios relating to noise emitting from the fans:
Please note that when the fan speed is constantly at medium or high, it’ll result in running at a speed not required for efficient working.
Switch to Auto Mode. This ensures the fan runs at a speed that is required as per the PM2.5 reading.
Sometimes, the fan speed may be constantly high, even in the Auto Mode, as a result of high PM2.5 reading in some environments.
You may try shifting the Air Purifier closer to a door or window so that the fan speed may change, resulting in slower speed and noiselessness.