Something is wrong with the design, the pump goes faster when the heater voltage is applied
Mains power is not fluctuating, checked at the hot air station and the mains socket, rock solid 242Vac, seems the voltage to the heater is pulsed (you can see the heater doing this) and the pump is just controlled via voltage like a light dimmer, but when the heater gets pulsed so does the pump, it makes a pulsing sound!
I've seen some tear down pics of this unit, very basic, looks as if it's all controlled by a very small Atmega on the LED board, no logic on the rear main board just triacs and optocouplers etc, 4 diodes for rectification (how much did they save by not fitting a rectifier lol)
Would love to see the insides of the black version of this unit or the CSI 825.
In fact, I think we should have a hot air thread with their clothes off, I want to see a Genuine Atten 850d and 852d, Aoyue 852A+/++ (Black version) Tenma 850d and REALLY want to see the insides of a the newer style Xytronic LF852D (Claims to have 40l/min pump and 600w heater)
Yes it's quite significant, it happens as the heater goes on and off and you can hear the pump change and the air flow changes dramatically.
I was doing some work on a motherboard and it was causing me issues.
If it has removable mains cable with IEC plug, try to change it. I've seen some fake cables with super thin wires which caused very high cable resistance. AFAIK that diaphragm pump is powered directly from the mains voltage through basically dimmer and there must be voltage sag somewhere. Did you measure the mains voltage in the same socket while station was operating?