When I received this rifle I shook it and heard a knock in the middle of the barrel shroud. I removed the shroud and secured the moderator baffles correctly.
Berserkeley Mike is probably right.
Oh, right, now I remember, mine was rattly too. One can't just stack the baffles up against the end of the barrel as then there is too much room. There is a screw that must be set (in the middle of the bottom dovetail), that the first baffle rests against, that keeps the stack tight up against the shroud end cap.
As for the puff, 13-14gr is a very heavy pellet in .177. I still don't really understand the Airforce valve, but imagine:
it gets knocked open by the hammer,
air pressure then builds in the breech, approaching that of the bottle, and at this point there is little force to close the valve,
that pressure moves the pellet down the barrel, which makes more volume for that breech air to go, which decreases the pressure,
and that pressure difference helps shut the valve, it moves with the airflow to the closed position.
A light weight pellet accellerates quickly and thus the valve will close quicker, meaning a smaller puff of air. But with a heavy pellet, is it possible that it takes a longer time to accellerate, that the valve stays open longer, and then there is more high pressure air that has to go somewhere, eventually out the barrel end, past the baffles and then out the side port, as there are o-rings blocking all other exits?
Do you get the same puff with a lighter pellet, say 7-10gr?
