Problems with Baxi Bahama 100!!?

Hi. I've recently had 2 radiators added to my central heating system owing to an extension. We haven't had any problems with our Baxi boiler for over a year now and everything seemed to be working fine. Although, it is working fine, every 4 to 5 minutes it is making a strange noise. As though it is letting air out. The noise is a large hissing noise as if it is revving up abit and then it dies down after a few seconds. Haven't a clue why it's doing it and the pressure has been consistently at 1.5 - 2 give or take abit (I wouldn't say it has dropped much really). Does anyone have any ideas as to what my boiler is doing every 4 to 5 mins??? Thanks in advance!