As there are so many ways to feed CO2, more info on exactly which set of reg, needle valve, solenoid, etc. may be needed to get closer to the correct answer. Meanwhile I might report on the only time I had a mystery noise from my CO2? Had an irritating "pop/click" sound that seemed to only occur when I was not near the tank.
Turned out to be the fish slapping and algea tab around!!!
Try to pin down that it is in fact the CO2, not some other random thing . Drove myself silly looking for the noise!