Tefal steam generator iron - steam leaking at the base unit, how can I fix ?

My Tefal Steam generator iron (the green one), switches on and generate steam. But, when I do Not hold the button down for the steam to come out of the iron ... the steam comes out of the bottom of the generator unit, and then starts to whistle. Water/steam comes through a hole of the bottom of the generator unit, and the whistling becomes louder and louder. Has anyone had a similar fault? If so, how do I fix it? It's a shame to throw it away as it has been a fantastic iron.