Cloister Bell

The Cloister Bell is an alarm bell located in the Cloister Room inside TARDISes. The Cloister Bell is to be used in the most serious and utmost emergencies. Its distinctive, sonorous ringing sounds resembles that of a large church bell. It could be answered by anyone inside TARDISes, and the Doctor told Adric that if he needed him, Adric could sound the Cloister Bell. (DW: Logopolis)

Instances when the Cloister Bell is heard

 * As a harbinger of the Doctor's impending regeneration. (DW: Logopolis)
 * When the TARDIS was trapped in the Dalek Time Corridor (DW: Resurrection of the Daleks)
 * When the Master opened the Eye of Harmony in an attempt to take the Eighth Doctor's remaining regenerations, signifying that the TARDIS itself was dying. (DW: Doctor Who: The TV Movie)
 * As the Doctor began to fly the TARDIS erratically (DW: Children in Need Special).
 * When the Master cannibalized the Doctor's TARDIS into a Paradox Machine. (DW: The Sound of Drums)
 * When the TARDIS of the Tenth Doctor and one of the Fifth Doctor crashed together. (DW: Time Crash)
 * After both a marketplace on the planet Shan Shen and the TARDIS exterior were affected by the Bad Wolf entity. The TARDIS may also have been warning the Doctor of the impending theft of the Earth by the Daleks and Davros, the merging of the Doctor and his then companion Donna Noble, or Donna's "death". This Cloister Bell could also have been warning of "The end of the universe", as the Doctor said at the end of the episode. (DW: Turn Left)
 * It is shown to be ringing in the advert for the 2009 Special, The Waters of Mars.

Other information

 * The Cloister Bell was not activated during an incident in which the fast return switch malfunctioned, threatening the TARDIS and its crew. It's possible the bell was deactivated by the malfunction, or had not yet been installed. (DW: The Edge of Destruction)

Behind the scenes
Note: It may be possible that when Carmen mentioned the Doctor's song ending and "he will knock four times", that she meant the Master was coming back.
 * The trailer for the 2009 special The Waters of Mars, played at the end of Planet of the Dead, included the sound of the cloister bell in the soundtrack. What significance this may have with regards to the story has yet to be revealed.