cycle skipping
The lower frequency you hear will be due to 'cycle skipping'
An inductor, by nature resists a change in current.
When you turn off the drive the inductor current falls, but may last much longer than you think.
So the next cycle, there is still current, and no current is commanded.
This is why we us high voltage to drive steppers so we can force the current to change in a shorter period of time, and hence run at higher frequencies.
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.