Worcester Greenstar Ri started whistling this morning?

Our Worcester Greenstar Ri boiler started making a rather loud high pitched whistling- type noise this morning.

On Saturday we changed one of the radiators and the CH was working fine afterwards. Sunday morning the boiler "stalled" - due to air in the system I think - but that was resolved and was working fine afterwards and yesterday. This morning, after it had been on a good 45 minutes I noticed a loud whistling noise coming from it. I switched the boiler off, and the pump etc - and the boiler was still making the noise. Checked the tank and it was full but topped up a little just in case. Have run the CH and bleed all the rads (no air in the system) and yet it still makes this noise. There's a little air valve at the back of water tank, which I have checked to see if there's any air trapped in there, but there's not.

The boiler is just over 2 years out - just out of guarantee by a few weeks, tut. Any ideas gratefully appreciated as to what else I can try? Thanks