what would be causing a "pouring" sound from my ferroli modena 80e?

A few months ago the over flow on my ferroli modena 80e combi was in constant over flow and the ch pressure gauge was over 3 bar. I shut the boiler boiler down and drained a few radiators around my house to bring the pressure gauge down to flat. I then filled the system back up to between 1 and 1.5 bar cold pressure, but since then the pressure seems to drop everytime I use the dhw. I've had to top it back up to between 1 and 1.5 bar pressure a few times and there now seems to be like a pouring sound coming from the boiler. What could be the cause for these problems?