Noisy Baxi 105he ?

I have a 3 year old Baxi 105he instant with a noise problem. After using hot water demand, eg shower, and the water is turned off, the boiler goes into a frenzy of gushing and squelching sounds for about 2-3 minutes. The pressure gauge drops about a third of a bar to .75 while this is happening. I have turned off the pre heat function on the boiler controls, as this used to cause the boiler to make the same noises, sometimes waking me up in the night! Any help would be greatly appreciated. Ta