It looks like many posters are interchanging vibration and ship's motion. The motions can be negligible or wild, for the reasons Skip lists. But the vibration issue definitely exists, at least IMHO.
Skipper, we were just aboard FANTASY (6670) for the 12/28 New Year's Eve cruise, and then a week later on DREAM (6690). Both cabins aft, Deck 6. On FANTASY, there was a definite consistent mechanical vibration. An educated guess tells me it was running gear related. There was no similar vibration in virtually the same location aboard DREAM. I had commented about it to DW while on FANTASY, and so we were both attuned to it when we were on DREAM.
I thought I had read elsewhere that there is a possible prop/shaft/rudder issue on FANTASY, which, given their sister ship status, makes sense. Hope someone else can confirm, I don't recall where I heard that, it may have even been from staff aboard DREAM.