Our toilet makes an annoying, low fog horn type noise every time we flush. The noise goes on the whole time whilst the cistern is filling up, except for the last few seconds. No other usage of water in the bathroom or anywhere else in the house causes a similar noise. The noise seems to resonate throughout the house.
What could be the cause of this and how might we go about trying to diagnose and fix it?
There is a recording of the noise here, though the droning noise seems quiet in the recording, as it's overwhelmed by the cistern filling noise: https://drive.google.com/file/d/0ByIxRsvAvipMdlNxMUhtT1dORFFyMTJvYmt5eGxfVVcxdmJj/view?usp=drivesdk