The problem started with the appearance of dust and grit falling into the bath. It was not at all obvious where it was coming from. With showers being preferred, the bath is little used, so this wasn't coming from the taps. I climbed into the roof space above the bathroom, but nothing was apparently amiss.
In the roof space there is an old tank, which I surmise was once the hot water tank. It has long been unused, and was presumably left there because it would be difficult to remove, and there was no compelling reason to do so. There is also some pipework that I had originally presumed was unused.
It's very dusty up there, and I have to wear a proper filter mask. As I was returning the mask to the garage area below the living area of the house, where I keep such things, I heard a noise coming from the hot water tank. Turned out there was a leak from where the heating element goes in. Given that the tank is under pressure, the leak was making a hissing noise.
Now, some years ago, I tried to trace the origin of the water-hammer noise, searching under the house without success.
My visit to the roof space made me wonder whether that pipe was really unused. Indeed it appears to have been sealed off. I now think that one part of it goes down the wall into the garage area to join the hot water output, with the other part heading down to the laundry. And where it goes down the wall is close to where the dust and grit is falling into the bath.
All of which makes me think that the dust and grit in the bath and the leak in the hot water tank are not unrelated. My theory is that this is the pipe that makes the water-hammer noise, given that it's not well attached to the framework, and that the tank leak changed the hammer pattern enough for the pipe to disturb different dust/grit in the roof space which is then able to enter the bathroom through a small gap in the ceiling trim.
On one level, this seems rather far-fetched. Yet the coincidence in timing is remarkable.
Sylvia.