Problems using an arduino with vex motors and motor controller 29
VEX motors with Servo library have the following functionality:

0 one direction (full speed)

90 stop

180 opposite direction (full speed)

So at 90, it is trying to remain motionless and hence creating that sound.

btw. You should provide power to motors separately from the Arduino. You might fry your Arduino if you run VEX motors from it. Best solution is to power using 7.2V 3000mAh Flat NiMH High Power Battery Packs.