Digital servos are over-zealous to hold their position by nature.
I've seen this happen with CA hinges and/or a little bit sloppy linkage.

Try putting a clothespin or just your finger's weight on the surface and if the shaking stops,
don't worry about it any more, it won't shake in the air. The load of air going over the surfaces will make it stop.

If the jittering continues with a load on the surface then get concerned.