As CO2 gets recirculated around the reactor it passes through the pump and impeller, this produces a 'ssshhh' noise and is typical and correct. If there is too much CO2 being supplied to the reactor the CO2 will build up
and this may produce a constant noise from the pump, as CO2 is constantly building and being pulled through the pump.

If there is a build up of CO2 inside the unit then it is possible either too much is being added or the bubble rate is excessive. If this is the case slow the rate that the CO2 enters the calcium reactor.