Most problems that pertain to aegs are usually preventable when it comes to taking boxes apart and fooling with the hopup.
The loud "popping" of the piston that you described is normal when a gun is upgraded. When a heavier spring fith much more force than the stock spring is put behing the piston it is naturally going to hit harder. In doing so, the piston is supposed to sound like it is hitting harder as well.
A loud and bad sounding box is also very preventable. This is mostly caused by overlooking the tuning of the motor when the gun is reassembled. It is totally wrong to just put a mechbox back in and throw the motor in after, and expect everything to mesh together as is should.
The motor has to pe perfectly positioned so as to make the gears mesh prcisely as to make the least amount of gear noise. This adjustment can be easily made by turning the screw in the bottom grip in or out until the desired sound is aquired.
Hop up problems are another story. If one were never to change a hopup it should still theoretically work. Most hopup problems are caused by either one of two things.
1- the use of wrong oil in the chamber, which causes the rubber to expand slightly and make the bbs hop almost too much.
2- Cleaning the barrel while the hopup is on, causing eventual wear of the rubber and possible failure to operate.
AEGs are somewhat semi complex pieces of machinery and it takes time and experience to learn the ups and downs of these systems. I have spent years tearing down boxes and putting new parts or other types of accesories into them, and the most impertant part that I learned was to have patience. Taking you time and making sure that you have everything right will save you alot of headache in the future, and you wiont have to do it all over again.