Could be the breech O ring. Mine in .177 had the breech O ring twisted and was loud and I could feel air coming from the breech. Pulled barrel and looked and could only see half the O ring. Pulled it out and it was intact but had half the thickness gone on one side. Replaced it and cured the problem. I think it came from factory this way as it was loud from the gitgo.
the puff might come from the exaust hole in the shroud
I have a exploded view on paper for a Blizzard (same action) I could fax you tomorrow. On second thought, I just remembered I have a scanner on my printer, if you would want that via email.