Hogfather - Sense

Death carefully put the instrument away and stood up. All was well, things were functioning satisfactorily, and

-but they weren't.

In the same way that the best of engineers can hear the tiny change that signals a bearing going bad long before the finest of instruments would detect anything wrong, Death picked up a discord in the symphony of the world. It was one wrong note among billions but all the more noticeable for that, like a tiny pebble in a very large shoe.

***

Hogfather - p46