I get more of a "Poof" than a report when I use my Darne 12 with Federal Featherlites, the low noise, low recoil loads. That gun was treated to a massive backbore at some point in it's life, however. It patterns better with hotter hunting loads.
I've also noticed that shooting in sub zero temps changes the sound of the report.
Don't have any idea about why on your gun, but, if it were mine, I'd keep it.

Best,
Ted