Whistling hot water tap

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Hi, can anyone offer advice on this? We have a 2 1/2 year old combi boiler (can't remember the make). Since last winter when you run the hot water tap on the bath the whole house rings with a whistling / squealing noise. We got a plumber out and he changed our diverter valve and turned the water pressure down, but the problem continues (and now it takes twice as long to run a bath and so twice as long listening to high pitched squealing!!!) We did install a new bathroom suite ourselves about 1 1/2 years ago - don't know if that would effect things. Help!