Foghorn

A foghorn was a sound signal used to alert boats to the presence of danger. The Fang Rock lighthouse was equipped with one. Reuben asked the Fourth Doctor to keep it sounding when the island was engulfed in fog, but he delegated the task to Leela, who found pleasure in its use. (TV: Horror of Fang Rock)

The spaceship Titanic was also equipped with a foghorn, in a possible homage to the original RMS Titanic. (TV: Voyage of the Damned)