Another possible cause according to Hi-tec, servo case screws too tight. If it doesn't go away after the first few flight (as mentioned above) try loosening the screws in the bottom of the servo case 1/4 turn.

From the troubleshooting section on Hitec's web site:
Q. Servo hums under load
A. This can be normal, the servo is trying to hold position against the force of a load. If it hums when no load is applied, try loosening the servo case screws a quarter turn.