We have now purchased three Blueair 511i Max air purifiers - one for each floor. I really like the size and the look of the units. We run them on the middle fan speed setting and you barely notice the sound. On the high setting, you can definitely hear them running. The removable, washable cover is a great idea. So far, my sense is that most of the dust gets caught in the outside cover so the main filter should last quite a while. It’s pretty easy to remove the outside cover and shake it out or blow it out using compressed air. These air purifiers are well designed.