Take this with a gain of salt, because some of it came from a CM on the Fantasy. We got off the ship last week after the 11nt Southern. We were midships, deck 9 family verandah. We're Platinum and have sailed all 4, so know our vibes lol.
Yes, there's the usual thruster vibrations, but they are recognizable. The other vibration only comes when the ship is running at or around 21kt and it's continuous.
Now the CM rumor. The Fantasy does have drive line issues, such that the Dream and Fantasy will swap itineraries at some point. The Fantasy has significantly more wear on her drive line that the Dream, so they'll switch, so that the Fantasy can put less stress on the drive. AFAIK the Dream & Fantasy have fixed pitch props.
That is all.