The wifes 69 Fastback ( I hate the sportsroof thing) , has a mild vibration under rpms up until about 5,000 . The engine is a rebuilt 302, with a mild cam. It did it before AND after the rebuild. We thought maybe the harm. balancer. Nope. The crank was balanced out. The fan is new, and spins steady. Its the original 69 302. Ive heard that some of the old 302s and 351s will do this, and only God can make it stop,even after a rebuild. Maybe if we make it an "Eleanor",,OKAY GAH IM JUST KIDDING !!