The sound is from your mechbox. Think about the gears, and the mechanics of what happens.
I have put up question posts about quieting your gun, Amos & Thundercactus set me straight if I remember correctly.
Its all about the MASK Piston/cylinder head set. Sorbo pads etc.
EDIT:
http://airsoftcanada.com/showthread.php?t=110748
http://airsoftcanada.com/showthread.php?t=107548