I have the same leak on my ego 06 mate.
After taking the gun apart and putting it back together again a stupid amount of times, i think i have traced the cause of the leak to the exhaust valve that sits at the front end of the rammer housing.
I found this out by doing the following, try it to see if this is the same problem on yours.
Remove the bolt from your marker so you can see the rammer bolt through the hole in the bottom of the bolt chamber.
Just in front of the rammer bolt you will see the end of the exhaust valve sticking out of the small brass guide.
If you apply a little pressure with an allan key or screw driver to the end of the exhaust valve while you have your air bottle on the gun you should hear a change in noise from your air leak, which i assume means you need a new exhaust valve kit as the one on the gun is letting air through before the rammer bolt strikes it.
Just ordered one for mine so will let you know how it goes.
Ash