I wanted to connect the fan to 3.3v, to reduce noise; as described by Hypher.
I only reconnected the + / red wire to 3.3V.
Later on I discovered that the fan was still turning on/off (which was not my objective 
). It appears that the fan is controlled via it’s GND connection
I can imagine that a lower fan speed will result in a lower air flow. Might result in the sensor seeing less particles per unit of time; measured values might be lower than 5V fan.
But because the measurements seems to be optimistic (see post 72 in this thread), I don’t think the lower fan speed should be an issue.