Other items to look at are the slop in the control system - clevis to control horn, clevis to servo arm, and even flex in the pushrod itself. I'm working through that issue right now in a Sig Kavalier where at above 3/4 throttle it'll get a loud vibrating sound that sounds like a very mild flutter. I've just about isolated it to the old style elevator pushrod which appears to be made of either hard balsa or very soft pine (the rudder pushrod is spruce). I'm going to replace both of them with a graphite arrow shaft style push rod besides sealing the control surface gaps with covering material and using high-end clevises.

Hogflyer