Need Help with Strange Vibration

[Seadweller]

Thanks guys...What puzzled me is that the vibration isn't there 100% of the time. I've been in the car several times trying to replicate it, and it never appeared, which is why I questioned him to begin with. This time, I was in the car with him for over an hour, and for nearly the entire time, there was no vibration at all. I was about to refer him to a psychiatrist, when the vibration just appeared out of nowhere while sitting at a light.

Would a bad motor mount cause an intermittent issue like this, or would the vibration be present 100% of the time?