Hideeho
From experience, I can tell you exactly what that is. You are running the avid or jc bell cranks & hit something. This cracked the top of the case where one of the pins that holds a gear stack goes in a hole. If you have it on & move the servo horn, it will move just a little bit while it is sitting @ rest. This causes the servo to bounce back & forth as it tries to find center. If it is under warranty, send it in & then sell it. If they won't fix a 6 week old servo, I'd buy a case & then sell to switch to a different brand. It will break in the same place again. I went through 3 cases for a jr 9100s before giving up on it & getting a radiopost red (this for my agama, but it also broke the case in the b4.1xl with the avid bell cranks). I have had no problems out of my Savox 1258 with the avid cranks.

Oops, this is what happens when you don't read through before responding. See above