[Plane Wrappers]

Vuster has the most correct answer. The counter balance is too large on the rudder and also the trailing edge is to thin and the air flow cavitates coming off the rudder. The simplest fix is to slice a peace of large fuel tubing straight down the center, slide it over the trailing edge and wick medium CA down the inside of the tube.
Then let it dry overnight.
Had the problem on both of my superXtras with 4 - 8611'a on them and this solved the problem.