An annoying sound

I must have been asleep for an hour or so. What had woken me? It was an insistent and ominous hissing noise, like the sound of a half submerged ballcock in the cistern. I dragged myself out of bed and padded through to the bathroom. No problem here; but the sound had followed me and was at the same intensity.

“It's obviously the overflow tank in the loft.”

Then followed thoughts of an inevitable …