h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

It's important to note that the devices that emit high-pitched sounds to scare off the rats can also be heard by your dogs, your neighbor's dogs, and so forth. And these may howl or become miserable, so think twice if you have a dog or live in a high population area.

Rats make them, too.

Our garden is infested with rats. We have a cat, but she only goes for the tiniest ones. I have seen her pit her wits against one more than 3" long and it was a much harder fight.

Anyway, once a friend was staying overnight, sleeping in the lounge, and he went out onto the terrace after everyone had gone to bed for a last smoke. Then he heard this repetitive squeak - he thought it was something from outer space! It was a rat being caught by the cat - presumably signalling to its tribe for help or as a warning, but certainly out of fear. It s a very high pitched, repetitive "peep", just like something electronic!