Noisy Central Heating System

Just had pump replaced (set on 3) a few weeks ago. potterton kingfisher, vented, 3 port valve in 3 bed semi. When on ch and hw the system runs fine, no noise coming from boiler or the system as a whole. When on hw only, theres a terrible racket with noise coming from the pump as if theres lots of air in the system. what I dont understand is if there is air in the system, why doesn't it happen when both ch & hw is on? Any ideas?

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reevesie
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