I would suggest getting someone Corgi wise out to it asap to have a look.
Its possible if the boilers room sealed that one of the seals is not making good anymore and the whistle is the air passing it. which if its the case either means fumes are entering your house or is at least At Risk of fumes entering the room.
Its possible if its a combi its a water noise, there are some boilers where o-rings ect can cause funny noises. but id get it checked just to be sure
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