What you are hearing might be some steam escaping through the vacuum breaker valve as the steam boiler pressure begins to rise. It would sound like "ssssssssssss" for several seconds, build in intensity, and then suddenly stop when the pressure became high enough to close the valve. My WEGA used to do this. At one point I had to replace the valve because it wouldn't close all the way.
I don't know if this is what you are hearing, though. Can you describe with more specifics? (Power on? For how long? Where are you in the light sequence? etc.)
When I installed mine, the power was off when I first applied live water pressure. All was quiet. Next, I turned on the machine, and could hear the pump start running as the boiler filled. All was quiet again, and from the lights, I could tell that the group boiler was heating. Several minutes later, the main steam boiler began to heat. As it finally started to build pressure, I could hear the faint sound of the valve letting some steam escape until it closed. A couple minutes later everything was up to temperature and pressure.
Has yours acted differently?