We have been in 8656 and 8658 for both Eastern and Western on the Fantasy. There have been times when we have felt the vibration and on the this past Eastern (at Thanksgiving) the last night it wasn't the vibration it was more of the ship movement that bothered us.
We love these 2 cabins. Right off the staircase, easy access to Cabanas and the pool, easy access to Animator's palace. Laundry is almost directly on the other side of the ship. And as the dad controlling the purse strings, these cabins are a fair distance to the shops! LOL.
These are the only 2 cabins we would book going forward. I wouldn't change a thing.