Our robot is up and running but the motors are making a buzzing sound, what can I do about this problem

Is your robot currently programmed so that it's being remotely controlled? If so, the problem may be that the joysticks are slightly off centered and sending a small amount of power to the motors, but not enough to make them move.

If you're using the older Radio Control Transmitter, you can adjust the "Trim" directly on the controller.

If you're using the newer VEXnet joysticks, you can recalibrate them: http://www.education.rec.ri.cmu.edu/products/teaching_robotc_cortex/index.html

yes it is remotely controlled, we just calibrated the remote and the sound is still there

I believe it will make a noise regardless, even when using sample programs that don't use motors, our robot still buzzes. I find it useful, as soon as you're connected you'll know.