W-Bosch 24 Si c/h boiler with the rumbles?

My 24 Si boiler; morning start-up. Kicks in then starts noisy rumbles. Pressure build-up has rapid fluctuations at above 3.5 bar. Turning on hot water tap relieves the rumbles. After about 30-45 minutes - works normally. When pressure is relieved to 1 bar the system repressurises to the higher level. The ext.filler link is disconnected & int. facility not fitted. What is most likely cause & how do I remedy?