why has the noise changed on my tefal steam generator?

This is my Fourth steam generator,first two had fixed water reserves, third a removeable one and now back to origional model, all this over 5 years, good but expensive unreliable irons! Now am increasingly worried that this is a dangerous noise-usually it makes a clicking then a 'generator'-type noise on the water transfer but now it sounds like a very loud boiling kettle,only FIVE months old at 1-2 hrs usage per week .