Soda fountain noises

Every now and then I visit a restaurant in which the soda fountain releases a burst of gas at fairly regular intervals while dispensing soda. The sound is like the violent hiss of a truck’s air brakes (tho not nearly as loud). The noise doesn’t come from the dispensing nozzle;it seems to emanate from the array of bottles and tubes next to the soda fountain.

What’s this noise? Venting CO2? Why?

Possibly a bad high-pressure regulator on the CO2 system. It can’t maintain the lower infusion pressure (around 80-100 psi, IIRC) from the high-pressure tank (1200-2500 psi?) without venting excess pressure. (Complete TWAG…)