Is it a vibration or is it tail wiggle? If it's vibration, it could be that the blades aren't balanced, bent shaft, flybar shaft not centered, flybar weights not equally out on the ends of the flybar, etc. If it's tail wiggle, it could be your gyro gain needs to be turned down ever-so-slightly.

You said you've really upgraded it, did you by chance install a brushless system? If so, these will cause tail wiggle unless you turn the gyro gain down.</p>