I just returned from Emerald Princess today. I was at the Terrace pool in the aft section of the ship. It bounced up and down for about 20-30 seconds. We thought it was the engine starting up - it was around 3pm or so, thus we were puzzled. A maintenance worker peered over the edge of the ship and walked away, not appearing to be concerned.
BTW.........the cruise was awesome. I have to admit that I didn't like the ship at first, but I ended up really liking it. This was my 16th Princess Cruise.
Best,
Mike