DRV8825 (just like Pololu ones) stepper driver, plugged into RAMPS 1.4 board drive 1.7A 4-wire stepper, both with or without microstepping.
At all times, stationary or moving the stepper produces considerable hissing resembling that of an amplified computer sound card background noise.
Hissing character changes if driver is touched on adjust pot and some other parts by finger or screwdriver. Have 10 drivers, all hiss. Tested on 2a and 30a 12v psu with no difference.
How can I find the cause?
When going slow through steps (full step, 2 sec per step) to check current between stepper-driver, multimeter shows ~1.7A on all steps. Shouldn't coils work in pairs, so current drops to zero on some steps?
Yes, that's hysteresis mode current feedback - it amplifies any noise in the current-
sensing components and the motor windings act a bit like a loudspeaker and produce
audible sound. Depending on how the chip operates and what frequency its working
on it can be more or less noticeable (some keep it ultrasonic and thus inaudible).
You should find that the choice of decay mode will affect the sound.
How are the wires configured - I would suggest separately twisting together each
windings wire pair, thus reducing the current interference between A and B windings,
which may affect the noise.
Endevour to keep the wires to separate motors separate from each other and other
cables - although this may make no difference its good practice.
To be clear about coils - should they be powered both at all times?
I had an assumption that they switch.