Brocock pistol, Grand prix 6, noise issue.

I believe my Brocock Contour is basically the same action and has an o-ring in the breech. My bet would be that the breech o-ring seal is missing and needs replaced. Brocock is not good at telling customers the size of the o-rings. My gun is a .22 cal and a .177 might be a different size. Hopefully someone will know the size and chime in. It might be helpful to know if it is a .177 or .22.