My RIGOL DG1022Z waveform generator made a lot of noise when I first switched it on. There’s a little fan in there, operating on 12 Volts.
Having opened up the device, I took out the fan and tried to operate it on 9 Volts. That still worked adequately, and made significantly less noise.
Running on 9 Volts it draws around 50 mA, so the resistance is about 180 Ohms (9 / .050). The feed line from the circuit board carries 12 Volts. To bring the voltage for the fan down to 9 Volts and maintain the 50mA, I needed a total resistance of 240 Ohms (12 / 0.50). So I used a 56 Ohms transistor I had around put it in series with the fan; that reduced the sound neatly to an acceptable level.