At first startup, AP550 may have some slight and soft "particle" sounds. That's normal, since our filter may need some warm-up time. After one to two minutes of startup, that "particle" sound should disappear.
However, If the noise continues, then please try the following:
Dirt resists airflow through the filter and may result in noise in the device.
Please use dry tissue/soft cloth to clean the two thin tungsten wires gently.
If the noise still continues, please follow the full cleaning steps of your Air Purifier by following the instructions on this link "How to clean your Air Purifier"