Baxi Combi boiler 105e making noise when I run hot tap?

Hi there - am hoping someone can answer this. The boiler seems to be working ok and has been for the 6 years it has been in. I keep the pressure topped up between 1 and 2.5 bar but recently I have noticed that it makes a "cascading water sound" whenever I run the hot tap in the kitchen (which last for another 10-20 seconds after I switch the tap off). I'm pretty sure it never used to do that. Does anyone have any ideas what might be causing this?