So you crashed your vehicle ripping off the left side of the truck and now it won't steer straight? With that sort of impact you could have damaged the servo or you still have some damage/bent parts on your truck. Disconnect the linkage from the servo (so it's just the servo with the horn on it) and check its operation and centering. If it doesn't center this way then the servo is damaged and you need to send it in. If it does center, then the problem is not the servo. You have something else in the vehicle that is bent or broken.

The hum is normal for digital servos. I take it you are new to this technology. It's just the digital circuitry refreshing to hold center (300 times/second). There is nothing to worry about there.