A friend of mine had a car that made a vibration. It turned out to be the CV shafts weren't perfectly straight. In the manual, they talk about runout being measured at the midway point of the shaft.

I assume your tires are balanced. Do you have a wheel or more that don't run true (are they bent from side to side)? If you but the wheels from front to back does that affect it?

Get a trunk monkey to ride up with the engine and see what he says.