Jonathan,
the motor will run quieter at a lower voltage, this is typical with stepper motors.
also you are running the 752 at 70V, this is the absolute maximum for this driver.
I would advise to run it at a lower voltage.
60V is a good voltage with the SY60.
As i mentioned before the problem is not with the driver but the 3V is too low.
I come across this a lot when people are using the parallel port.
It is a very cheap and simple 5 minute fix.