The vibration that the original poster was speaking about was the one I was referring to and this is due to the thrusters. Perhaps on your cruise there was a related vibration to the propulsion problem. This was not the vibration the OP was speaking of nor the one I was answering the question about. This vibration has been on all the C class ships and it shakes the whole aft area. I know because I was in one of the aft cabins. In this case we are both correct.