Perhaps understanding what is going on will help you troubleshoot more:

The servo has a little motor inside and a little knob attached the the gears that tell it what position it's in. When the servo gets a signal that tells it to be in a new position the motor starts up until that position is reached then stops. If the arm gets moved then it's not in the right position anymore and the motor comes back on. Sometimes when some linkage binds up the motor turns on and pushes the linkage, but then when the position is found and the motor turns off, the linkage doesn't stay in the new position and goes back to the old. This can cause the motor to turn off and on to make a buzzing sound. If your linkage can't travel to the end of the servo travel then going to one end or the other would cause the servo to try to go to a position it can't physically be in which causes it to buzz.

To resolve these problems you want to make sure that the linkage can physically move to the very end of the servo travel or you can limit the servo travel if you have a computer radio as others suggested. It is always better to fix the linkage by using more or less servo arm leverage because you don't loose some of the servos resolution when you limit it's travel and because it doesn't depend on a computer radio and it's configuration.

If the servo buzzes while in the middle of the travel then your linkage is binding up. You need to make sure everything moves freely and smoothly.

To test if the linkage or the servo itself is bad simply do as Ken suggests and remove the servo horn and see if it still buzzes. If it does then for whatever reason the servo can't place it's arm where the signal is telling it to. If the buzzing goes away then it is working correctly.

It is important to fix servo buzz because it can overheat and damage your servos and because it will greatly reduce battery life which usually results in a crash unless your paying very close attention.

Hope that helps,
schu