Re: Tortoise switch machine noise
Just a WAG. Many power packs do not have pure DC. The DC many have do not use filter capacitors so the DC is really pulsing at a line frequency. It would be 60 hz for half wave rectifiers or 120 hz for full wave rectifiers.
I installed a 4700 ufd capacitor across the terminals of a power pack to better filter the DC. My way.
BUT, I had to make sure the power pack reverse switch could not be moved. I actually wired around the switch so I would not blow up the capacitor by reverse voltage.
Again, my way. Your mileage may vary. I also reduced the power pack voltage to 9 volts.