This is somewhat similar, Mike Mayberry is one of the top fellows at Hitec......Not saying Mike knows about Futaba equipment but I have seen this even on my JR 4131's with NP.
http://www.rcuniverse.com/rcarchive/149/2002/04/2/19441
A list of issues can be found on this:

Servo Buzzing - Servo buzzing is a condition where your servo "buzzes" while sitting still. There are many reasons for this, most of which are not correctable.

Tight Dead Band - When a servo is designed, the dead band is made as small as possible. The dead band is the amount of stick movement that is required before the servo begin to move. It cannot be set to "0" or else the servo will buzz, but if it is to wide then the stick response is not good.

If you have one servo that buzzes periodically then it probably is a "tight dead band servo" and there is nothing you can do about it. If all of your servos buzz then you probably have a high battery or receiver overdrive condition.

High Battery Voltage - Some radio's will buzz, when the battery has just been charged, and the buzzing will go away after a few minutes of operation. You must remember that when we first take our batteries off the charger, a 4.8 volt receiver pack will measure about 5.8 volts and a 9.6 volt transmitter pack may measure as high as 11 volts. This overcharge condition is normal and will bleed off to the normal voltage rapidly. This high voltage coupled with either a tight servo deadband, or possibly the following receiver over driving condition may cause buzzing and is nothing to worry about..

Receiver Overdriving - Our receivers are expected to work beyond our range of sight, yet we also expect them to work when our transmitter is 1 foot away from the receiver. This is asking a lot, but most units do handle it. If your servos are buzzing, try moving your transmitter about 10 ft. away from the plane and see if the buzzing stops! If it does, don't worry about it. The Automatic Gain Control (AGC) circuitry is just being over driven when the transmitter is too close. This may only happen in conjunction with the overcharged batteries.

Dirty Pots - Dirty servo pots (feedback potentiometer) will normally show up as jitter on the servo or jerky operation. Since most of the time your servo is sitting in neutral, vibration may cause wear on the pot surface. This will show up as jitter when the servo is sitting in neutral. Usually when this occurs the pot face also gets dirty and the servo responds unevenly to stick motion. (Like little jerks getting to where its going).

If your servo jitters in neutral try moving the trim lever to one end and then the other. If the jitter stops at one extreme or the other then you probably have a dirty pot. The jitter stops because you moved the pot wiper arm off the worn center spot.

When jitter occurs due to a dirty pot, the only good fix is to replace the feedback pot. Cleaning seldom lasts long and on most of the new servos the pot is a sealed unit.