Noisy Boiler

Recently my Apollo boiler has started to make a noise very similar to that of an electric kettle just as it starts to warm the water - like a hissing type sound. It only makes this noise when it is set to the CH&HW position on the boiler. It does not make this noise when switched to HW (Hot water). I have recently changed the downstairs rads and added TRV's. Last year I flushed the system with a boiler restorer and replaced the pump. Since the installation of the new rads I have added a prohibiting treatment.

The boiler thermostat is on High
The Room thermostat is always 22oc

Does anyone know why the boiler is hissing? Is there some kind of bleed operation on the boiler itself required? (I have l bled the rads and pump)

Have you got the pump on high if not turn to high, have you got a bypass fitted or leave 1 rad with out trv's, i would say you have a flow problem

sort it out before you get lime build up in the heat exchanger