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Hi you need to test the gas flow rate and emmissions on the boiler it could be to much gas entering the boiler causing it to whine
Or if it sounds like a kettle heating up and it only happens when its getting hot it could be Scale on the heat exchanger. you will need to run descaler through the boiler to clean it out.
Lastly if its a mechanical whistle it could be the fan.
I dont know what type of whistle it is but these three options are common.