There is a hissing (Air leak coming from the trigger / Bolt area ETHA2

Hi, there is an air leak coming from somewhere in my Etha 2 it sounds like its coming from the bolt area or just behind the trigger. Ive taken the entire thing apart and watched the youtube tutorial. all orings look fine however on my work station i found a very small o ring that i think fell off somewhere but i cant figure out where. Help?