The quietest aeg that you can make is most likely something like an Aug or previously mentioned an F2000. The gearbox is very enclosed and the areas around it can be padded out with foam and etc for further sound proofing.
Although that's only if all variables between all the different aegs are constant.
Noises come from every single part of the gearbox. The motor, gears, piston, piston head and cylinder head slapping and other bits.
The fact is if you take 10 aegs out of the box. All the same model and from the same manufacturer you will find each one sounds different