I don't know if the RR works the same, but I had a problem like that in my nustang and it was the balancer. It is a weight to balance the engine, atteched to the engine pulley. In the mustang it is made of three parts:a pulley attached to the engine around that is a rubber and then the weight. In my case the rubber part was falling apart and the weight was moving around causing the engine to be out of balance.
Hope this helps.
BTW I put on, one made of steel so it won't happen again.