here's the reply I got from Rocket ;
If there is steam coming through the group it maybe that there is some air inside the heat exchanger circuit. Generally this will bleed through as you use the machine.
If you very carefully loosen only the nut on top of the group with a hot machine. USE A TOWEL AS THE WATER WILL BE UNDER PRESSURE AND ONLY LOOSEN THE NUT A HALF TURN OR SO. Once the air bleeds out and only water flows from the top of the group, tighten the nut.
I just tried it. Seems to have fixed the problem.
When I turned on the machine so I could bleed it, as it was warming up, I heard that weird sound; can it be caused by air within the HX circuit ?