It really depenends on the PM sensors you use. For the Plantower 5003 we use in our monitors, you only hear a faint noise from the fan when you put it directly

It really depenends on the PM sensors you use. For the Plantower 5003 we use in our monitors, you only hear a faint noise from the fan when you put it directly at your ear. However some other monitors need an extra fan in the enclosure and some of them can get noisy. For CO2, the Sensirion SCD41 is a good sensor which uses photo accoustic principle. We found NDIR sensors using light spectometrie to be a bit more accurate like the SenseAir S8. Price wise the two are quite similar.