Boiler has started making a strange sort of low vibration/whistling sound this evening. I say boiler, but the noise remains when the boiler is switched off, so it's a noise coming from the general direction of boiler and related pipework. It disappears when you turn the hot tap on, but remains if the heating is on.
It stops every now and then but then starts quietly and builds up and gets noisier.
Any ideas?

