When you pull on the string, the latch goes straight up with a sudden noise. When you are twiddling it, or twisting it round the pin for a protracted length of time the latch rattles, so the person knows you are coming. The cats always knew when a person was walking past and pausing, or walking past and stopping to open the door, because as soon as the latch started to rattle, there was a half dozen paws reaching out from under the door...

LTS