Thanks for the suggestions. I'm using a safety spinner nut instead of a cone-spinner. I also had an SK90 engine on this model before the OS and it also had the same vibrations in the tail. Maybe it's just something with this model (Big Stik 60)? I was worried that something might be loose internally in the OS Engine. But doesn't sound like you guys think that would be the problem.

I called TowerHobbies where I bought the OS engine. They said it's under warranty and I can send it in for testing/evaluvation/repair. Do you think it's worth doing?