your anti vac valve is sticking, you can open the top and press it back in but they only cost aout +- $12 to replace and yo might find it now will stick a lot, maybe take it out and descale / clean it, but i reckon the oring in it might be old @ 10 years.
goggle "anti vac" it *should look like this, maybe next to the pressure relief valve
I call the problem "false pressure" on my machine i turn it on an leave the steam wand open in the morning. When i hear *it gurgling and a bit of steam i just shut the steam off then i hear the sari go "click" and its happy days :) maybe i should spend the $12... I had a laugh i am cheap
PS I am assuming your machine has one of these, but i reckon it will.....