As usual, I argue.
The AEG's loudest sound is from it's gears. As TacMaster said, it is not easy to build one with low noise, but it is sure possible.
Then the next loudest can be the sound of piston hitting the piston head. This snapping sound comes from the barrel. With proper barrel length and piston head, it can be lowered, and then if you install a suppressor, this snap is transformed into a thud, that sounds much quieter.