worcester 24i rsf fault with heating?

After heatings been on for around 10 mins starts to make noise like air in the boiler ,Changed the pressure valve,Repressured the tank via schraider valve,still the same, as noise starts if you turn on hot water domestic side the noise stops then starts again as pressure builds up. Raidators are hot and no air in them .ANY IDEAS AS TO THE PROBLEM WOULD BE GRATFULLY RECIEVED