Stepper Motor noise when not moved
2nd edit Unfortunately this chip has this bug with some motors due to higher L/R ratio under no load causing the alternating cycles to commutate at half speed which becomes audible.  EMI can add to some noise if the current sense track is parallel to the flow of current instead of ideally at right angles.  So TI decided to integrate the sense R's inside the DRV8886 and others with autotuning the PWM shutoff decay time rather than fix the problem in the DRV8825,
Hybrid decay time should be best but many other users in TI's forum had the same issue.  I might have tried adding 1 Ohm series R to each coil in hybrid mode to see how it affects performance as this reduces the L/R time constant but maybe not enough to get below 15us to prevent /2 frequency commutation.

Edit:  The glitch may be  noise ingress issue on your open collector PWM signal but the obvious issue now is 14.7kHz PWM on idle. 

This is audible and worse than old TV flyback transformers with NTSC that ran near this rate and some people could hear this.

Correct this by raising the PWM frequency up by 50% or so. 

( if it were only possible arghh)


Most likely Vss or Vdd glitch noise crosstalk to Vgs , so put low ESR e-cap near driver and use twisted cable to reduce CM noise. Also test with earth ground. Does stepper get warm during idle?

Scope Vdd-Vss with ground clip removed using barrel =1cm gap on 1:1 probe Ac coupled into 50 ohm DSO termination for best transient noise capture. Or just coax.

If this is OK then it is not conducted feedback but radiated feedback to driver inputs. Use minimum loop area on motor current and driver input signals or STP cable and/or with ferrite CM choke suitable for L/R in this range. By low ESR caps I suggest &lt;=50mohm. This is a common issue.