Yup, the toilets are always one of the things the first comedian act of the cruise hits on. If you have ever flown and used the bathroom on the airplane, its very simliar. When flushed, the vaccum makes a large whoosing sound and almost a pop. The big joke is that if you are a big person, never hit the flush button while sitting down or they will have to come to your cabin and pry you from the seat. (Either that our you will walk away with a very large hicky where you don't want one.
In reality its not really that bad, but it does take the unsuspecting person by surprise.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013