Sealed system noise

[alarge]

Hi
I wonder if you might be able to help?
I have a sealed central heating system with an expansion tank type of arrangement. It is about three and a half years old. Recently a strange whistling sound had started to come from the water tank area. It seems to start mainly when the hot water side is on (the hot water light on the control panel is on) and then the heating is switched on. I have charged the expansion tank so that the pressure gauge is between 1 & 2. I have also made sure that all of the radiators are bled.
Do you have any suggestions that could stop the noise?
Regards
Allan Large