Dagu Rover 5 motor whistle

I recently bought a Dagu Rover 5 with two motors and encoders (not yet connected).  I'm using a DFRobot Romeo to control it and using the supplied 8-cell battery holder to power it.

Though I haven't done much with it yet, it's been fun.  My question is the (acoustical) noise that the motors make.  Whenever power is applied, a low-pitched whistle comes from the chassis, presumably from the motors.  I have found a couple of online Rover 5 videos that confirms it isn't just my chassis that does this.  Still, it doesn't sound like a normal sound.  Does anyone know what causes this and whether it's something to worry about?