DIAGNOSIS That's almost always the filter. If it's not seated perfectly flush and square in the base, or if the pre-filter wrap is bunched up, the fan will vibrate against it.
FIX Power it off and unplug it. Lift the unit off the base. 1) Take the filter out completely. 2) Check that the gray pre-filter mesh is wrapped smoothly around the whole cylinder with no big wrinkles. 3) Look inside the empty base—make sure there's no debris like a Lego or a big dust bunny down there. 4) Re-seat the filter. This is the key part: lower it straight down, don't angle it. You should feel it settle all the way into the grooves with a soft 'thump'. It has to be perfectly level. Put the unit back on and power up.
IF THAT DIDN'T WORK If the noise is more of a high-pitched whine or grind, the fan motor bearing might be failing. That's less common, but it happens. You'd need to contact Blueair support for a motor replacement or repair.
HEADS UP A very faint hum or 'whoosh' is totally normal on higher fan speeds. The rattling we're fixing here is a distinct, obnoxious vibration.