Water / heater help

hi all, new to caravaning and getting stuck with water, boiler issues. I have a swift challenger sport 442 with the truma  combo boiler. I follow all the instruction, fill water tank then  attach, run hot then cold on taps until flowing steady just on wafter pump, finally switch the boiler on. All ok then half hour later go for a glass of water or wash my hands and water blow out / air lock and sometimes consistent low level noise from boiler or pump area. Must add have checked water container full ! Any advise would be great.