well i am pretty much a noob in airsofting, but ive got

 TM P90 tr
 a silencer with an SRC M4 barrel running through it
 two 9.6 1100mah batteries

 no real upgrades to the gearbox other than an SRC piston

compared to the other numerous guns ive shot, and heard fire, my p90 is almost whisper quiet.  

im comparing mine to a CA M4 w/ silencer,  CA G36C w/ silencer,  SRC M4 w/ big QD silencer TM MP5k w/ silencer and various unsilenced Mp5s and other aegs

i never noticed how quiet it was until i let someone else shoot it