Take off and landing!

I have a SIME super plus 90 combi boiler. The problem is that when the boiler kicks in after a few seconds it starts to make a noise like a plane taking off. Once it reaches the correct temperature the noise subsides as if the plane has passed over head. It runs at about 80degrees C. When it reaches 60 it kicks in again and the noise reappears.

Is this air in the system? If it is how do I get rid of it? Please help this is driving me mad!!

Forgot to mention when I switch on the hot wazter tap the noise goes away. The pressure is set at 2 BAR when hot.

Sounds like a blockage on the heating side of the boiler, Is the pump running or is the diverter valve working, ie the radiator getting hot, could also be the by-pass blocked or not open. The noise is usually the result of the heating not being able to get away fast enough ie. transferd to the water and pumped around the heating system