Strange boiler noise

[longshanks2]

I have a conventional CH system, A potterton prima boiler, 7 rads, HW cylinder.
When the system kicks in I get a "glug, glug, hisssssss " noise coming from the boiler this seems to continue for a while the tends to settle down, I have regularly bled all rads during the winter CH months, I even manually operate the auto air valve, I have loaded the system with the appropriate chemicals, I have bled the pump. This has been going on for about a year now I now need help, have I missed something?. Listening to this noise close up to the boiler it sounds like water is struggling to get to the heat exchangers in the boiler.
Can anyone advise me please.
regards

 

[carlramsay]

how long have you had the boiler, could the heat exchanger be getting scaled up.

 

[carlramsay]

is it struggling to get to the heat exchanger or struggling to get through it ?

 

[longshanks2]

The boiler is about 10 years old, I can't tell weather the water is struggling to get to the exchangers or away from it. The best way of describing the noise.
When the system fires up as per timer setting ie, 7am, the boiler hums for about 30 secs then there is a gluging noise as if water is entering an empty pipe then it hits a hot surface and I hear a long hiss, pauses then repeats a few times. I had a service done last year by the CH fitter of boiler, he said there was air trapped in the heat exchanger and recommended a chemical additive, that has not worked.
regards