It probably switched to a higher fan speed, likely Turbo mode. Check the Fan Speed Indicator on the control panel.
Press the Fan Speed Button to cycle through Low, Medium, High, and Turbo. Set it to a lower speed for quieter operation. In Auto Mode, the fan will also get louder if the sensor detects poor air quality.
If that didn't work: The noise could be physical obstruction. Ensure nothing is blocking the air intake (front) or outlet (top). Also, make sure the unit is on a hard, flat surface—not carpet—and has 12-18 inches of clearance from walls and other electronics.
Heads up: If it's a grinding or rattling noise, unplug the unit and check that the front panel is latched shut and the filters are seated properly. Never run it without filters.