Nope, not a clue. But if this was the problem, would symptoms not be bad running, smokey exhaust etc? Engine runs fine and pulls cleanly.
The rattle is not always there, if it was the water pump, wouldn't it be continuous?
Same for other suggestions, it doesn't rattle all the time and it happened suddenly, it's not grown from a slight rattle to a big one. Surely if it was wear, it would be more gradual?