If you have a problem with only one pistol cylinder I would say it is a problem with its valve closing. The opposite can also happen when transferring too much liquid CO2 when you get valve block and need to use the venting cap. Any residual gas trapped between the extinguisher and cylinder when the lever is closed is at about 820 p.s.i. so it does make a noise.

Baz (see pic)

https://i.imgur.com/1ADwNgG.jpg