DC motor with high pitched sound

Hi forum

I just tried to hook up a number of small to medium sized DC motors to the PWM output on my new Arduino board. Most of the motors are pulled from old printers.

Ecerything works fine, but all the motors emits a fairly high pitched sound, which is very irritating. When connected directly to a battery or other DC source this doea not happen.

Since all the motors have the same pitch, my gues is that there is something in the motors that vibrate at the frequency of the PWM signal itself - could this be true ?

Is there a way to get rid of it ?

Does all notors do that ?

Thanks

MikMo

Ups! i forgot to mention that i hooked up the motors via a TIP120 transistor excatly like here:

http://www.tigoe.net/pcomp/code/archives/bx-24/000294.shtml#000294

MikMo