ATM motorizing scope questions

["Jerry B. Hillman" <JBHillman@ev1.net>]

Hi all,
Since the military has seen fit to keep my son overseas, it is up to me to
figure out how to motorize my scope.  Not wanting to be encumbered by a
computer and all that entails, I decided to build a simple motor drive
system.
Months of researching the electronics involved in controlling a stepper
motor system, versus a dc motor system led me to decide to use the dc motor.
I finally found a simple circuit to control the motor. Pulse Width
Modulation.  Now, I have very little electronics experience, and zero
experience in putting a circuit board together.  However, after about two
hours of soldering, preceeded by numerous hours on the internet learning how
to read the circuit diagram, and all those little symbols, I was ready to
test it.  I hooked up the motor, then the battery, turned it on and
proceeded to test the system.  It worked first time.
Now, two questions.  I first noticed that the motor produces a tone that
increases in pitch as the motor speeds up.  It disappears completely at
about 3/4 speed(except perhaps in the range dogs can hear. Any answers as to
why this happens?
Secondly, how much torque should the motor produce.  Even at full speed, I
can stop the motor shaft with my fingertip, although it is a little
difficult.  At it slowest speed, it is quite easy to stop. My understanding
was that PWM control didn't lose torque at the slower speeds.  That was one
of the reasons I chose this control system. This is a 6volt motor running
off a 5 amp battery.
Any of you electronic experts out there that can answer this would be much
appreciated.
Thanks, Clear skies,
Jerry