I have a 66 225hp coupe,4speed,standard rear gear that has a vibration somewhere in the drivetrain in the first 3 gears.This sounds and feels like something unbalanced in the engine until I shift to 4th gear and it is gone,every time.Irebuilt the engine 25 years ago for the man that owned it at the time and it was that way then,I recently got the car and it hasn't changed.Car is very stock,only change is a very mild cam.I remember having to put a new cylinder liner on the hole that had dropped a valve and boring it .030 over.Everything else normal stuff,no trans work,Ijust put axle bearings in this week,rear end orginal,driveshaft splines a little loose,u-joints tight,no mounts bad that I can see.I am at a loss,has anyone else had this happen?Thanks in advance.