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Heatslave 20/25 subtle humming from boiler new motor fitted fully serviced recently, it's the sort of sound you would get from a HV transformer that sort of hum deep and annoying, nothing to do with circulation pump it's still there with the circulation pump off. I guess it only leaves the oil pump but this part is fairly new too. I read that high water pressure can cause this although I'm sceptical and wouldn't know how, it's currently at 1.5bar.