Vaillant TurboMax Combi hissing noise

My Vaillant Turbomax combi is eight years old. When the central heating starts there is a hissing boiling noise from the burner chamber. It sound like the water is boiling in the heat exchanger pipe. It lasts for a five or ten seconds then settles down and it is then silent. I\\\'m sure it did not used to do this. Any ideas?

The heat exchanger may have slight scale build up and causes a kettling noise. You can add a silencer chemical but i beleive this just gets rid of the noise doesnt actually get rid of the scale which is very difficult.