Volkera compact 29A Combi always making kettling noise?

We have installed Volkera compact 29A Combi. I switch my boiler from hotwater supply to central heating.It starts to make a noise;as if a kettle is boiling or the boiler is flooded.My overflow doesn't drip at all ,but the noise in the boiler can be heard outside coming from the flue
Does anyone know how to stop this noise or do I need to get my central heating powerflushed?