stepper motor making weird noises...

So I've finally gotten my unipolar stepper working!
Before it did not work too well, going in random direction, making
funny soundes etc...

So here is what I've observed and am still having problems with.
Currently the only way I can get the motor to go a nice speed is to
ramp it up from maybe 50Hz and then ramping it to a few kHz. However
in the mid range frequency the motor starts to make weird nasty
sounds! what could be the reason of this?
also is there a reason why I have to ramp it slowly? I feel like I
should be able to get this guy up and running pretty quickly, am I
wrong? Help!!

.

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