The engines are not at the rear of the ship anyways. And its not like there are huge outboard motors strapped to the ship! Modern cruise liners are diesel electric. They use giant diesel powered generators to power electric motors (isopods) that turn the props. So being on the rear of the ship is no more noisy than anywhere else. If you stand up on the very top deck under the funnel you can hear the ventilation fans running and you may hear some vibration noise but that's about it.
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