Worcester 24i Screeching/Whistling Constant?

We have a Worcester combi boiler fitted on our central heating system.

We have had noises in the past which I wasnt happy about, they was sort of rattling noises within the boiler when it was turned on which settled after being on for a while. The plumber came to have a look and said there didnt seem to be anything wrong.

Anyhow, tonight, the central heating system has been on for a couple of hours and now and again I heard a screech which I thought was the battery low indicator on our smoke detector which it wasnt after the following happened. Then all of a sudden about an hour ago it is a constant high pitched sound , at first it sounded like a fax machine but now its one constant level.

We turned the heating off but the sound hasnt stopped even though the heating has.

The pressure gauge seemed to be okay (between one and two).

Please tell me what this is and that my boiler isnt about to explode.

Just as additional information...

The boiler is around five years old. We had new radiators fitted and the new boiler back then, there is some old pipework that still remains though.

We have had a few air problems and have to bleed the radiators every now and again and have also mentioned that to the plumber too.

A month or two ago the pressure kept dropping and I informed the plumber who had a look and said he couldnt find anything wrong (no leaks) and that it could be an air problem causing something to kick in (to do with the overflow). It seemed to settle and other than the bit of rattling noise every now and again (once its been on for a while that stops) it seemed to be okay until this.

I have even tried turning it off at the plug and this doesnt make any difference.

I really want this noise to stop, I have a two year old in bed and hes disturbing because of it :(