Plaza deck is deck #5, low, but not too near the engine room. There is one deck of cabins, the Contenental deck, below you (decks 1 to 3 are crew only).
Plaza deck is also one of the decks occupied by the main restaurant, in the stern.
There have been a number of complaints on cruiseboards about vibration in the restaurant from the engines/propeller shafts/propellors/whatever, below.
The good news is that the cabins on Plaza deeck are towards the front of the ship, where vibration would likely be a lot less.
As for "traffic" I'd say you have a very quiet part of the ship.
One potential source of noise is the cinema and theatre, immediately above. This could be an issue if you are early birds. Check the daily sheet for show times!