Glowworm 30cxi thumping/puffing?

Hi, I just moved into a house that has a fairly new glowworm 30cxi boiler. It works fine most of the time, but quite randomly when someone's having a shower, it'll start making a thumping / soft puffing noise and you can see the steam venting from the flue in soft puffs. When it starts doing this, the green led also starts flashing, but stays green. Any help would be great. P.S the pressure is a steady 1.0 - 1.1