Updated my A4988 Drivers to DRV8825s today and went from 1/16 to 1/32 microstepping. I can now hear a really high pitched noise (which most people probably wouldn’t pick up) after moving any of the steppers which stays on until I disable all steppers even when they have all stopped moving.
Any idea’s what’s causing that and if it can be avoided? It’s quite annoying 
That would be the motors oscillating. A “microstep” is essentially the driver chip telling the motor to step, and then almost immediately telling it to step back. It does this faster than the motor can respond and so it sits there “mid-step”. The number of “psuedo” steps you can create between two actual steps are called “microsteps”. And the more microsteps you try to put between two actual steps, the faster the driver has to switch back and forth between step forward/back, and that shows up as an audible whine as the motor is trying to what it is being commanded to do.
This is more cost effective than putting a 32:1 gearbox on the stepper (where each actual step would cause the output of the gearbox to go 1/32nd of a step) but such systems are silent as the motor is never in an intermediate position.
Motors machined to tighter tolerances whine less (less wiggle room) and cost more. They are some times marketed as “high precision” motors to reflect the additional work.
This is pretty common with DRV8825 drivers. A higher supply voltage can push the frequency out of the audible range, but most people either just live with it or switch back to Alegro drivers.
seeing as 1/32 does not increase your actual resolution at all over 1/16 I suspect you may prefer to go back to allegro. The whining is pretty normal for DRV8825.
Thanks all. Is configuring them to 1/16 microstepping likely to affect the noise?
No, it’s a product of the chopper frequency.
1/32 microstepping will give you smoother (and thus possibly quieter) motion, but asn @Wolfmanjm says, it doesn’t increase your resolution, at least not in any useful way. There’s not really a good way around the idle hum, so if it annoys you that much, you’re probably better off switching back.
@Mark_Rehorst This is about the hum while the motors are idle, not the noise while they are in motion, which should be lower with a DRV8825 than any of the Alegro drivers.
@Mark_Rehorst This is actually incorrect… the maximum stepping frequency on Smoothie is 100KHz, but unless you run a feedrate at that speed it will not be sending 100KHz to the drivers. eg running 100steps/mm at 100mm/sec feedrate the stepping frequency is 10KHz to the drivers not 100KHz (that is both on Marlin and Smoothie). So this has nothing to do with the maximum possible step frequency.