Answer by Thomas Zerbarini:
The front behind the cockpit and passenger doors. At high speeds, wind noise around the passengers doors and exits can be loud.
Also, many cockpits have windows that can open when on the ground. These cockpits tend to be louder.
Faster cruising aircraft have much louder front ends as well. For example, the Boeing 727 is considered a very fast airplane capable of cruising above .90 MACH (90% of the speed of sound). At these speeds, the air just outside the cockpit actually reaches speeds above the speed of sound and are breaking the sound barrier (even though the airplane is still below the speed of sound). Therefore, the noise level increases dramatically in areas of the airplane that reach or exceed the sound barrier.
Here's a good video about shockwaves and the sound barrier…
Rear fuselage mounted engines also make the front of the airplane more quiet. The farther away you are from the engines the quieter the experience.
I think the most quiet seats on any airplane are the forward, lower decks of the Boeing 747 and Airbus A380–center seats.