PWM and motors

I'm trying to control a toy car that uses dc motors using a L293d (as in http://www.instructables.com/id/Control-your-motors-with-L293D-and-Arduino/step3/Arduino-code/ ). However when I try switching to PWM, the motors don't move at all and I get this odd humming noise coming out of my arduino.

What could be the problem? (And am I going to break my arduino??)

odd humming noise coming out of my arduino.

Not good.

It sounds like you have wired it up wrong. Are you suer that you have ground common between your arduino and battery?

Yeah I'm pretty sure I've got it wired up correctly. It works using normal digitalWrite() after all.

Interestingly enough, some of the pins have different frequency hums. :P

EDIT: It was do to the duty cycle being too low. Whoops.

Nothing to see here....