Diagnosis
The noise is almost always caused by the pre-filter cloth sleeve being loose or improperly aligned. At high speed, airflow vibrates the fabric.
How to Fix It
Turn the unit off. Remove the front cover. Check the fabric pre-filter sleeve. It must be stretched taut over the main filter, with the sewn seam running vertically. Ensure the fabric handle at the top is centered. Smooth out any wrinkles. The bottom elastic band must be seated fully in the groove at the filter's base.
If That Didn't Work
The noise may be internal. Unplug the unit. Gently tilt and rotate it to listen for anything loose inside, like a detached screw or piece of plastic from shipping. Contact Blueair support if you hear an internal mechanical rattle.
Heads Up
A consistent, smooth whooshing sound is normal, especially on setting 3. The rattling is distinct and sounds like something is loose.