Analog servos may buzz with some radios if the frame rate is too high. For instance, some radios allow the user to select a frame rate that is 22 mS long or 11 mS long. If the shorter period is selected, the pulse stretching circuit does not have time to completely discharge, and so the servo will just hum and buzz. The faster frame rate works great with Digital servos, but not with analog.

It would be more useful if all the information is provided, as to radio type and servo type as well as type and voltage of the airborne pack, because that too can cause problems with some servos.

Of course the suggestions to check the freedom of the linkages is valid too. But if all the servos buzz, then I would look in another area.