Hi Krysta
Removed servo arm. No humming.
There does not seem to be any binding or out of alignment in the linkage.
Adjusted (lengthened) linkage till there was no hum.
Now have droupping E with linkage connect point .21" aft of hinge line right angle measured from the hor stab.

I previously gave the CA hinges over 100 full deflexion flexs to loosen them up.

Q1) Is the Sub trim function possibly applicable as a solution?

Q2) Would I accomplish anything by moving the E to a VR Sw to test if it hums in different set positions other than trying to hold a set position with the stick?
Q3) Would reversing the servo temporarily to see it hums aid in solving problem? .Tested makes no difference.

Recentering the servo? Elaborate please I do not understand. Is that a physical or electrial recentering? It's mounted exterior aft +/- 12" from hinge line, not touching the fuse all sides, with a ply block behind aft mount holes.