There are two things that will cause that, either the upper case, or the gears. If yours is one of the newer models, its most likely the gears, cause the case has been updated.

I had one with the shakes, and it worked fine as long as there was a load on the servo, it only had the shakes when it was off the ground. I ran it for a whole season like that!

I ended up buying the replacement parts and selling it to a guy with full disclosure that it had 'the shakes'. He replaced the parts, it fixed the problem, and still works to this day.