Out of Time

The out-of-time sound is used to indicate that the episode has ended for the night. Any contestant who is still playing has to come back the next night. In the UK, it's called the klaxon. In the U.S., it's called the horn

Moments involving the noise
Contestants like Stephen Fox and Paul Cleary have been interrupted by the noise just after completing Fastest Finger. Contestants like Steve Devlin and Ingram Wilcox have been interrupted by the noise after winning £500,000 and had to wait until the next night to see the million pound question.