Worcester 28cdi boiler makes a squeeling niose?

I have a Worcester 28cdi which squeels loudly when domestic hot water demanded. It starts off quietly then squeels as the water heats up. It seems to be related to temperature and increasing flow.
Water to water heat xchgr is changed as well as the red pressure vessel with the schrader valve. All indications are normal. Pressure is steady at 1 and1/2 - 2bar.
I suspect the diverter valve. Can anyone confirm this please?