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. The feed line from the circuit board carries 12 Volts. To bring that down to 9 Volts I needed a resistor of around 60 Ohms. I used a 56 Ohms transistor and that reduced the sound neatly to an acceptable level.