My 2007 Nissan Quest has an intermitten vibration at around 35 to 45 mph. Nissan techs cannot determind cause since viration is intermitten.
An intermittent vibration can be caused by any one of a 100 things. Engine performance problem, transmission shift fault (assuming automatic), suspension component wear, borderline wheel balance problem, etc.
You might provide some info about whether or not any codes have been present, how many miles on the vehicle, condition of tires, etc, any history of collisions, what has or has not been done to solve this, etc.
Van has 45,000 miles on it and the tires are in good condition. No collisions. Nissan techs have driven van but no vibration occured while they tested. No codes.