Stepper Noises

Hi All,
I have been test running my steppers on home brew drivers based on
the design on Steve Pilot's Web site. They are 6203 4A based chopper
drivers. What seems odd to me is the high pitched squeel i am getting
from the steppers while powered. Is this normal for a chopper drive?
I am running them at 24V from a toroid transformer (filtered), The
steppers are Nema 23's rated at 2.2V x 3.7A 1.8 degree/step.
I assume this is due to the choppers frequency that it chops at, but
would still like to verify this with more seasoned cnc veterans.

TIA
Tom S.